representing its condition in the same units being respectively for the four years, 7, 13, 19, and 13, whilst it descends to I in the fifth year. Now, for comparison, suppose 9 units as the initial capital, and the same relations of expenditure and profit, we shall find that the diminution does not commence till the fifth year, the growth thus continuing a year longer, the figures being 15, 24, 35, 36, and 33 respectively.

These two cases, in which the quantities are of course merely arbitrarily chosen for example, and in which the ratio of expenditure and profit as to increase is exaggerated, suffice to demonstrate the principle, which may be applied to organisms. It is because the calf at birth is a much larger animal than the lamb, having been carried longer by its parent, who from her greater size could of course give to the offspring a greater proportionate amount of living matter to commence life with, that the cow lives longer than the sheep, or rather inherits a later natural limit to life. The quality of the germinal matter and many other conditions which have to be provided for, in laying down such rules as this, by the expression 'cæteris paribus,' must always be taken into consideration.

We have, then, seen reason to think that the duration of life, after growth is completed or coming to an end, depends on the amount of living matter accumulated during growth, and that this depends on the size at birth, cæteris paribus. Thus it is that we may trace

the rationale of that connection between time of growth, time of gestation, and potential longevity, which has been pointed out,' though we can see no good reason why the number 5 or any other number should express the ratio for a whole class of animals.

6. Elements of the Life Period.

It is obvious that by increasing the duration of any one part of the lifetime of an organism, the sum total or whole may be lengthened, and it may be useful, therefore, to consider what are the phenomena which most clearly involve periods of time for their manifestation in organisms.

They are, 1st, the period of evolution (including in this term both growth and development); 2nd, the period through which reproductive activity is spread; 3rd, the period of dissolution or decay. Any condition of existence which necessitates the lengthening or shortening of one of these periods for the requirements of the organism directly affects longevity. In the period of evolution are included the period of embryonic development, of fœtal life and incubation, the larval period, which many animals exhibit, the period of growth as indicated by osseous completion or similar perfection, or the assumption of the mature

1 Both by Flourens and Buffon, as well as the earliest writers on longevity.

state. To the period of dissolution belong those slow senile changes, or that rapid exhaustion of power, which it is clear from the preceding section depend on the period of growth, in large measure as to their length, but which are also affected by external agencies, which will be referred to again. The period of reproductive activity is a variable quantity according to the case, in some lasting but a brief day or hour of the whole life, whilst in other organisms, for certain definite objects, it may be prolonged over a considerable time, recurring at intervals. This period does not seem to depend directly on the period of growth at all, but it is distributed to organism with regard to other and special conditions.

7. High Individuation favours Longevity.

Having seen, then, that the period of evolution is a very important item in longevity, since (p. 44) the period of dissolution depends directly on it, as to duration, we are in a position deductively to make the proposition that potential longevity varies with, or is favoured by, high evolution, if it be admitted that high evolution, i. e. complex structure and large bulk, both or either, are identical with evolution occupying long time. A few considerations will prove that this is admissible. You cannot have mere growth of bulk without time, as is obvious from general facts; the very word 'grow' implies

not a sudden but a gradual acquisition of volume. We infer that a great thickness of strata, or of other such accumulations, has taken a long time to grow, measuring the time by the bulk; similarly, a large population or a large city is known to take a long time to accumulate. That complexity and interdependence of structural arrangement also implies time spent in the evolution of those arrangements, is proved from the observation of other cases of non-organic development. A language, a civilization, a city, a land-surface, the sidereal system, are universally admitted to have taken time in their growth in proportion to their complexity, in proportion to the extent of redistribution and readjustment of parts of which they bear tokens. And so we may infer that high organic complexity and high organic bulk, both involved in the term 'high evolution,' or better perhaps 'high individuation,' postulate time in proportion, and we may conclude that high individuation favours longevity.

8. Small Expenditure favours Longevity.

We now come to a proposition which we have already anticipated in previous paragraphs, and which has great importance, qualifying largely as it does the application of the preceding, and explaining the reason of the existence of a normal potential longevity limited in some cases abruptly by natural

It.

decay, and in other cases only remotely so. is the influence of expenditure on longevity which really gives rise to that complication which led us to distinguish normal and absolute potential longevity. (See antea, p. 25.) Expenditure, which was pointed out as increasing more rapidly than the means of supplying the loss it involved, and thus limiting natural life, may be increased greatly or diminished in organisms according to their requirements, and may be so far diminished as never actually to allow that period of balance and the subsequent one of decay, which we saw occurred in one group of organisms, to come on in another.

Expenditure is of two kinds-that involved in the wear and tear of obtaining and assimilating food, and generally carrying on the life, and that involved in the propagation of the species by the elaboration or separation of living portions of the parent organisms. We may distinguish these as―(1) personal, and (2) generative expenditure; and we may affirm, that by diminishing or increasing either of these, you favour or antagonize longevity. This follows deductively from the conclusions arrived at with regard to the relation of organic structure to longevity. Expenditure of either sort uses up the matter of life, and hastens on the period of natural decay. It is observable that these two forms of expenditure are naturally, inter se, antagonistic, on the old principle that 'one cannot have a pie and eat it.' If personal