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is a difficulty which Professor Ray Lankester expressed so long ago as 1876, when contrasting Mr. Spencer's theory with that of Pangenesis.* And now we are certainly entitled to go further, and to enquire why the effects of constantly performed mutilations, which are not repaired, do not tend to become inherited after many generations, as they certainly ought to be on Mr. Spencer's theory. Finally we ask—and ask in vain-for an explanation of the fundamental facts of atavism.t

Mr. Darwin's "Provisional Hypothesis of Pangenesis" differs very fundamentally from the theory we have been hitherto considering. It distinctly recognises the necessity of grappling with the two questions which we have seen to be the fundamental problems of heredity-the problems of transmission and of development-and it answers these questions in a clear and unmistakable manner.

Mr. Darwin supposes certain organic particles called gemmules to be the specific bearers of hereditary tendencies. These are continually-at all stages of development-being given off by every cell in the body. The gemmules are continually circulating throughout the body, and finally collect in the reproductive cells. Each gemmule is a representative of the cell from which it took its origin (at that period of its life-history), and is capable, under proper conditions, of developing into a similar cell. The germ-cells, then, being simply collections of such gemmules, possess the potentialities of new organisms exactly similar to the old. This is Mr. Darwin's solution of the problem of transmission. The problem of development he solves as follows. gemmules representing any given stage of development have a special affinity for the partially-developed cells of the stage immediately preceding. They seek out and unite with these, and are thus able to develop in the right order their corresponding parts. This extremely ingenious suggestion Mr. Darwin supports by reference to the unerring accuracy with which pollen of the right species alone develops pollentubes when a number of different kinds are placed on the stigma of a flower.

The

The phenomena of atavism, Mr. Darwin explains by supposing that some gemmules lie dormant for

"Advancement of Science," pp. 280-282.

It is hardly fair to Mr. Spencer to charge him, as is so often done, with being too vague and general in his theory of heredity. In the first place, he is not in the least vague, and the impression that he is usually arises from failure to thoroughly understand his meaning. That his theory is "general," is true, as we have already insisted: but it should be remembered that he expressly says "a positive explanation is not to be expected in the present state of biology. We can look for nothing beyond a simplification of the problem." It is, however, quite open to us to criticise the theory on the ground of its being inconsistent with known facts; and further, to demand now a fuller and more special explanation of the phenomena of heredity, as the result of the many years of research which have elapsed since Mr. Spencer's words were

written.

"An organic being is a microcosm-a little universe, formed of a host of self-propagating organisms, inconceivably minute and numerous as the stars in heaven" ("Variation under Domestication," vol. ii. p. 399).

more or fewer generations, and suddenly, owing to especially favourable conditions, are enabled to develop into their corresponding structures.

It will be seen that this theory explains the problem of transmission mainly by the hypothesis of the redevelopment of new gemmules from the cells of the body into which the former gemmules were developed. It explains the problem of development, by supposing that there has been a preformation* of distinct germs of structure, and that by means of the presence of these the germ-cell is able to produce the organism.

Its

Mr. Darwin's theory explains with great readiness most of the observed abnormal phenomena of heredity, but it was to explain the supposed hereditary effects of use and disuse that it was especially constructed. This was indicated in the last paper (p. 210), and it was there pointed out that Mr. Darwin was right in his belief that a theory on the lines of Pangenesis would have to be ultimately adopted, if acquired characters are inherited. explanation of the problem of transmission by, the assumption of the redevelopment of the gemmules from the cells of the body alone enables it to explain the hereditary transmission of the modifications ac quired by those cells. The other theories of heredity which profess to allow of such transmission-such as Mr. Spencer's, depending on assumed absolute solidarity of the organism, or the late Professor Nägeli's, depending on cyclical development of idio, plasm, which we shall consider presently-are open to even more serious and fundamental objections than is Pangenesis. We are, I think, in a position to say that if acquired characters are transmitted, a theory of heredity not essentially differing from Pangenesis will have to be adopted, and the difficulties-very great difficulties-attaching to it will have to be explained away somehow.

The theory of Pangenesis has been often and severely criticised; it is not my purpose to give a list of these criticisms here, but I shall consider three which seem to me to be of special importance. First, there is the argument about the inconceivable number of gemmules that must exist in the fertilised egg-cell

Harvey, following Aristotle, enunciated the theory of epigenesis, to explain the process of development of the fertilised ovum into the adult organism. He believed that the various structures of the adult were formed by the successive differentiations of a relatively homogeneous rudiment (the fertilised ovum). Malpighi contradicted this on the ground of having directly observed the body of the chick in the egg during the early days of incubation. He unjustifiably assumed that it existed in the egg as a whole before incubation. Later on his views were taken up and extended by Bonnet and Haller, and became widely accepted. Bonnet, however, modified his doctrine in later life, and looked upon the egg as being only an "original preformation" of the body, not necessarily an actual miniature of the latter. C. F. Wolff, in 1759, entirely exploded the theory of "evolution," or preformation as generally understood, and re-established "epigenesis;" but Darwin in his theory of pangenesis undoubtedly revived the conception of the preformation of distinct germs of structure, though not, of course, the idea of a miniature organism in the ovum, but rather a conception akin to Bonnet's later views. It is in this modified sense that we use the word "preformation" and contrast it with pure "epigenesis," to which Professor Weismann has conspicuously returned.

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of, for instance, a mammal. This is an old objection to Pangenesis, and one to which Mr. Darwin replied by pointing to the enormous number of molecules which probably exist in a cubic millimetre of water. But when we consider, first that a gemmule is very much larger than even a very large molecule of a complex organic substance, and secondly try to get some conception of the enormous number of cells which will have to be represented, the objection appears of considerable force.

The second argument I propose to consider is that furnished by Mr. Francis Galton's and Professor Romanes' experiments on the transfusion of blood and transplantation of tissues in rabbits and guinea-pigs. Mr. Galton performed extensive experiments in transfusing the blood of distinct varieties of rabbits. He obtained no hereditary effect. Mr. Darwin would not admit that these experiments negatived the doctrine of Pangenesis. It was no essential part of his theory, he argued, that the gemmules should use the circulation as a means of transit. Obviously they could not do so in many of the lower animals and in plants. But in reply we may argue, with Professor Weismann, that it is difficult to see why the gemmules should fail to take advantage of so favourable a means of transit, and also how they could contrive to avoid it. Professor Romanes has since repeated these experiments with great care, and has also carried out very complete skin-transplantation experiments on guineapigs of distinct varieties, again with negative results. Now in this case, on the hypothesis of Pangenesis, a certain number of gemmules must have been present in the transplanted strips of skin, and how they failed to produce any effect is not at all clear.

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Perhaps the most trenchant criticism of Pangenesis is Professor Weismann's, and it is best given in his own words. It is a criticism of Darwin's solution of the problem of development by the hypothesis of preformation. "One and the same part of the body must be represented in the germ or sperm-cell by many groups of gemmules, each group corresponding to a different stage of development; for if each part gives off gemmules, which ultimately produce that part in the offspring, it is clear that special gemmules must be given off for each stage in the development of the part, in order to reproduce that identical stage. But the ontogeny of each part is in reality continuous, and is not composed of distinct and separate stages. We imagine these stages as existing in the continuous course of ontogeny ; for here, as in all departments of nature, we make artificial divisions in order to render possible a general conception, and to gain fixed points in the continuous changes of form which have, in reality, occurred. Just as we distinguish a sequence of species in the course of phylogeny although only a gradual transition, not traversed by sharp lines of demarcation, has taken place, so also we speak of the stages of ontogeny, although we can never point out where any stage ends and another begins. To

imagine that each single stage of a part is present in the germ, as a distinct group of gemmules, seems to me to be a childish idea, comparable to the belief that the skull of the young St. Lawrence exists at Madrid, while the adult skull is to be found in Rome. We are necessarily driven to such a conception if we assume that the transmission of acquired characters takes place."*

These three lines of argument show the heavy batteries which can be brought to bear against Pangenesis from various sides. We can hardly accept such a theory unless no other is possible.

Various attempts at modifying it have been made, of which, perhaps, the most important is Mr. W. K. Brooks'. I shall not, however, discuss it here, but refer the reader who is interested in the matter to Professor Weismann's very complete criticism of it ("Essays on Heredity" pp. 326–332). (To be continued.)

A

ROSSENDALE RHIZOPODS.

No. 6.

LL the Rhizopods figured and described in my previous articles, have belonged to the suborder Lobosa-having lobose, or finger-like pseudopodia, and containing many forms familiar to most microscopists. I have now to describe some less familiar Rhizopods, belonging to the sub-order Filosa. In this sub-order, the sarcode is not so obviously separable into a clear ectosarc and a more granular endosarc, but apparently consists wholly of the latter; and probably on this account the pseudopodia, instead of being lobate or finger-like, are in the form of exceedingly delicate, forked threads, which become finer and finer as they branch, and are usually more numerous than those of the lobose Rhizopods. The sub-order contains seven genera, five of which are represented in this district, and about seventeen species, some of which are beautiful objects, the tests being made up of round, or oval, over-lapping plates, arranged in such a way as to form definite patterns.

Pamphagus mutabilis, perhaps the most interesting species of the genus, I have not yet met with; but another species, P. hyalinus, is not uncommon in our clear ponds and wells, though from its minuteness and inconspicuous character, it may readily be overlooked. On a side view it is sub-spherical, and the lower end is produced into a short neck, with a circular mouth, through which the long, fine pseudopodia are protruded. As generally seen, it appears as a roundish granular mass, greyish in colour, occasionally with cloudy yellow patches in the interior, and in specimens from some localities

"Essays on Heredity," pp. 316-317. "Law of Heredity," 1883.

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