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Reptiles living in hot countries, and feeding on large masses of food at intervals, have small expenditure and live long. The higher Reptiles are the most sluggish and inert of any animals in proportion to their degree of development; and hence, their expenditure being small and their development high, we should expect them to exhibit great longevity, which they do. A very instructive contrast is afforded by Birds and Reptiles, which are so closely allied in structure. The active, expending Birds are short-lived as compared with such Reptiles as the Tortoise and Crocodile.
Echinoderms, being exceedingly sluggish, living on the most easily obtainable food, in many cases, viz. the organic matter diffused in sand, live longer than would be expected from their comparatively low place in the scale of life. Actinia, which is also almost like a vegetal as to absence of personal expenditure, as are other sedentary coelenterates, owes its great longevity to a relatively high evolution, in respect of integration. It buds never (or rarely), and breeds sexually but little.
The parasitic worms and crustacea might be expected to have a great longevity from the total absence of personal expenditure; but here, as in many plants, there is enormous generative expenditure, which shortens life, the small percentage of those born which ever get into the happy conditions of a stomach or gill, being the reason for this great generative outlay. Most of these forms die on reproduction. Tænia does not die at once, because of its tertiary aggregation; that is to say—it is separated into a number of joints, which, one by one, come to maturity and die, whilst new joints continue to grow from the head.
These are some of the most striking inductive verifications which the collected statements furnish; others are to be found by a further examination of the list.
II. Other Relations of Longevity. There seem to be some minor modifications of the terms 'evolution' and 'expenditure' which affect longevity, and which do not strike us at once as coming under those heads, and yet are very plainly influential in the result. How difficult it is to get clear views in the intricacies of such a problem as the one before us, we may let Mr. Spencer say, who shews, in his chapter on the Inductive Verification of the Laws of Multiplication, the frequency of complications which can only be dealt with by the use of the phrase 'cæteris paribus' as a continual qualification.
Animals which feed on large masses of food, of great concentration—as, e. g. other animals, or special fruits and portions of trees—are longer lived than those feeding on diffused and widely-spread food, as the lower sorts of vegetable growth and decaying material. This we see in the greater length of life of carnivorous and frugivorous animals as compared, cæteris paribus, with herbivorous and garbage-eaters. This reduces itself to a case of evolution and bulk; for in the first group it is an advantage to be large and highly endowed, to be swift and powerful, and to secure the whole mass by one effort. In the second group, five mouths will take in more nutriment than one, it being equally diffused; and hence it is better for a given bulk of the species to be divided into five small individuals than retained in one large one.
Where the acquisitive power increases more nearly with the bulk, as in vegetals, such a distinction does not hold.
It is in accordance with this relation of bulk to food that insects which feed on widely-spread vegetable juices, or similarly wide-spread garbage, are shorter lived than the birds which prey on the insects, or than other insects which are carnivorous; and that the lower animals, generally feeding as they do on diffused food, are shorter lived. Thus the frugivorous apes are longer lived than other animals similar to them in many other matters which are not fruiteaters; carnivora generally than herbivora, in the various classes and orders, cæteris paribus.
Another apparently important influence in longevity is what Mr. Spencer calls 'tertiary aggregation.' This is obviously only a form of increased evolution. Mr. Spencer supposes that what has been somewhat
unaccountably called vegetative repetition--for it is quite as truly animal or animative repetition-is an arrested production of zooids by budding; that it is, in fact, a merging of many individualities in one. This, we see, in many cases, like being compared with like, is accompanied by increased longevity. The Tania which exhibits this tertiary aggregation in a loose sort of way continues to develope from the attached head, and to protract its individual duration of life long after sexual organs have ripened and death followed in the separated proglottides. In the closely-allied Trematodes (flukes), in which the individuality is that of a secondary aggregate, death follows reproduction; as also in the Nematodes, which live in similar conditions, and are structurally allied to the other parasitic worms. In vegetals we find the same relation holding good, the multiaxial plants being the long-lived, the uniaxial the short-lived. That tertiary aggregation only has this accompaniment as long as it implies an increased comparative evolution, is seen in the fact that the Mollusca and Vertebrata, supposed to be secondaryl aggregates, are longer lived than many tertiary aggregates; whilst similarly the relation of expenditure to longevity considerably qualifies the influence of tertiary aggregation. The Annelids of high tertiary aggregation, i. e. of many segments (e.g. Lumbricus and marine forms), are
1 That is, aggregates compounded of primary aggregates; primary aggregates being simply cells.
much longer lived than the Naids and Chætogaster, with few segments and one year of life. So, too, the Echinoderms are tertiary aggregates, and consequently have long life as compared with simple Vermes. Tertiary aggregation acts in aiding longevity like the construction in five compartments of the Great Eastern steam-ship, if one is injured and lost, the others can go on without it, or even one may survive by itself. The question of tertiary aggregation brings us very near again to the discussion of individuality, which is not within our scope. Remembering what was said at the outset as to this, it is clear that tertiary aggregation acts by merging many individualities into one, and thus improving the chance of continued life.
Social organization is a sort of tertiary aggregation, in that newly-produced individuals do not separate from but remain attached to the preceding generation, supporting and ministering to the life of the older constituents. Thus it is with civilized man. He is supported in old age by the younger generations ; the hope of, and confidence in, such support which the younger individuals have, being the strongest bond of society.
1 Naids and Chætogaster are continually giving rise to new segments, which separate and become new individuals. How long the life of one original ancestor may be thus carried on by means of division is not known ; but it is probably not for the immense period supposed by some writers; for, when sexual organs develope, the worm ceases to bud, and dies.