REPRODUCTION 



653 



protoplasm concerned, bears the aspect of an im- 

 paired nutrition. In the Metazoa, although repro- 

 duction is not so entirely a mere process of cell 

 division as in the Protozoa, a connection between 

 nutrition and reproduction is o.hservable. The 

 common hydra, with an abundant food-supply and 

 favouring circumstance, grows rapidly, the growth 

 becoming a process of asexual reproduction and 

 taking the form of the production of numerous 

 buds, which may themselves produce a crop of 

 secondary buds. But if the conditions become less 

 favourable to nutrition through the lessening of 

 the supply of food material, or, in terms of the 

 more definite generalisation emphasised above, less 

 favourable to assimilation through, say, a fall of 

 temperature, then this rapid growth ceases and 

 reproductive organs are formed and sexual repro- 

 duction takes place. Planarian worms in good 

 nutritive conditions form asexual chains of daughter 

 worms. A check to nutrition is followed by the 

 separation and sexual maturity of the links. 



Fruit trees are root-pruned in order that the crop 

 of fruit may be abundant ; the reason being that, 

 as nutrition is lessened by such pruning, there 

 follows an increase of reproductive activity which 

 takes the form of fruit. If the vegetative activity 

 of the plant be what one desires, then the flower 

 buds are nipped off and sexual activity prevented. 

 A similar result follows from the castration of 

 animals. The position of the (lower at the end of 

 the vegetative axis is an expression of the fact that 

 at that point the food-supplies are more scanty 

 than at any other point along the axis. This dis- 

 tribution of food matter is shown again in such 

 plants as the tiger-lily, which have a mode of 

 asexual reproduction, one that is of continuous 

 growth, by the development of little bulbils 

 which occur in the axis of the leaves, such bulbils 

 Ix'ing only found on the lower part of the stem. 

 Other factors than the supply of food-matter influ- 

 ence assimilation and reproduction. As in the case 

 of all molecular movements, variations of tempera 

 tu IB are an obvious cause of change of state. For 

 every animal i.e. for every peculiar form of proto- 

 plasm there is a particular temperature which, 

 other things being constant, is most favourable to 

 rapidity of assimilation. This point is widely 

 different in the various forms of life. In every 

 case it is probable that a rise of temperature up to 

 a certain point is followed by a feverish state of 

 buily and a tendency to hasten sexual maturity 

 and leproduction. If our conception of the relation 

 of assimilation to reproduction be correct, then, as 

 already suggested, a fall of temperature lielow that 

 must favourable to assimilation ought to be followed 

 by an increasing tendency to sexual reproduction. 



Reproductive maturity the blossoming of the 

 individual life occurs, as we have seen, about the 

 time when growth ceases. In the lower animals 

 sexual maturity is attained relatively sooner than 

 in the higher forms ; but there are many strange 

 cases of precocious and retarded reproduction. 

 Thus we may contrast our common annuals and 

 the ' century plant ' or American aloe, or some 

 midges, worms, and even a couple of amphibians, 

 which are reproductive during larval life, with 

 highly evolved animals, such as the elephants. 

 The physiology of reproduction must take account 

 of that profound reaction which affects the whole 

 system as sexual maturity is attained, of the vari- 

 ous ways in which the reproductive elements are 

 separated from the parents, of the relation which, 

 alike in plant and animal, may be established 

 between the fertilised egg-cell and the mother- 

 organism, and of the way in which an embryo 

 thus nurtured eventually becomes independent. 

 Moreover, there are often highly evolved psychical 

 activities associated with reproduction notably 



the love between mates and between parents and 

 offspring. 



But, while reproduction is a blossoming of the 

 individual life, it is also in a sense the beginning 

 of death. The flower and fruit often end the life of 

 the plant. It may be that the processes of rupture 

 by which some of the simplest organisms reduce 

 their bulk and multiply their kind are but a few 

 steps from the more diffuse dissolution of death. 

 It is a fact that in some simple animals e.g. some 

 ' worms ' the parent, and especially the mother, 

 ruptures and dies in liberating the reproductive 

 elements. So, among higher forms, not a few 

 insects mayflies, locusts, butterflies die a few 

 hours after reproduction. The exhaustion is fatal, 

 and the males are sometimes victims as well as 

 their mates. In higher organisms the fatality of 

 the reproductive sacrifice has l>eei) greatly lessened, 

 yet death may tragically occur, even in human life, 

 as the direct nemesis of reproduction. In short, 

 the process by which new lives begin, by which the 

 continued life of the species is secured, tends to be 

 antagonistic to the life of the parent individuals. 

 The old leaves fall oil the tree, and their places are 

 filled by others. 



The Rale of Reproduction and Increase. The 

 rate of reproduction depends upon the constitution 

 of the individual organism and on its immediate 

 environment and nutrition. The rate of increase, 

 which is much more difficult to estimate, depends 

 upon the wide and complex conditions of life which 

 are often included in the phrase ' the struggle for 

 existence.' While it is true that organisms some- 

 times exhibit an extraordinary increase in numbers 

 in favourable areas and seasons, and while we 

 know of many forms and even of whole races 

 which have dwindled away 'and become extinct, 

 the fluctuations in the numbers of plants and 

 animals seem for the most part to be imperceptibly 

 gradual. Their rate of reproduction is adjusted to 

 the conditions of their life ; the rise or fall of 

 the population is seldom emphatic. The essay of 

 Malthus (1798), in which he showed that the 

 increase of human population tended to outrun 

 the means of subsistence, but was met by various 

 checks, afforded suggestions to Darwin and Wallace, 

 who extended the induction of Malthus to plants 

 and animals, recognising in their increase the 

 fundamental condition of the struggle for exist- 

 ence, and analysing the checks as various forms of 

 nut u ral selection. But Herbert Spencer's analysis 

 of the laws of multiplication was even more pene- 

 trating. Including under the term individuation 

 all those race-preservative processes by which 

 individual life is completed and maintained, and 

 under the term genesis all those processes aiding 

 the formation and perfecting of new individuals, 

 he showed both inductively and deductively that 

 individuation and genesis vary inversely. Genesis 

 decreases as individiiation increases, but not quite 

 so fast ; in other words, progressive evolution in 

 the direction of individuation is associated with a 

 diminishing rate of reproduction. 



The Importance of Reproduction in Evolvtion, 

 As almost every individual life begins in the 

 intimate union of two living units the male-cell 

 and the egg-cell there is in the nature of the organ- 

 ism's beginning an evident possibility of variation. 

 The two cells, and more especially the nuclei of the 

 two cells, are intermingled ; and in the vital com- 

 bination which results new characteristics may be 

 evolved, old features may be strengthened, peculi- 

 arities may l>e averaged off. On fertilisation as a 

 source of variation, emphasis has been laid by 

 Treviranus, Gallon, Brooks, and others, while 

 Hatschek regards the intermingling as an im- 

 portant counteractive of disadvantageous indivi- 

 dual peculiarities, and Weismann finds in it the 



