DISCOVERY 



217 



the hare is greater than could be explained if account 

 be taken only of difference in size. 



Again, the bat as a rule bears only one offspring at a 

 time, and is thus unusually unproliiic for so small an 

 animal ; and this is ascribed by Spencer to the relatively 

 high rate of expenditure resulting from the habit of 

 flving. He pointed out, further, that in comparing 

 birds of various species with mammals of the same 

 size and weight, the creatures which are continually 

 going through the exertion of sustaining themselves 

 in the air, and propelling themselves through it, are 

 less prolific than those whose movements are restricted 

 to the surface of the ground. Other instances of the 

 application of the same principle are supplied by the 

 domestic animals which, with few exceptions, have 

 larger and more frequent litters than wild animals 

 belonging to the same species. Thus the tame rabbit 

 will breed six or seven times in the year and may 

 have as many as eleven young in a litter. The 

 domesticated ferret is far more prolific than the pole- 

 cat, which is its wild prototype. The domestic sow 

 breeds regularly twice a year, and sometimes even 

 oftener, and has remarkably large litters, whereas the 

 wild sow breeds less frequently and produces fewer 

 5'oung at a time. The wild sheep has only one j'oung 

 one and a single mating season, while the domestic 

 sheep frequently produces twins or triplets, and, as 

 long ago Aristotle observed, " where the weather 

 is warm and fine and food is abundant may have lambs 

 twice a year." 



These and other examples of the same principle are 

 noted by Darwin, who attributes the increased fertility 

 of the domestic animals to a long habituation to a 

 regular and copious food supply without the labour of 

 seeking for it. In a similar way Spencer accounts for 

 such well-known facts as that hens cease to lay when 

 they begin to moult — " While they are expending so 

 much energy in producing new clothing, they have 

 nothing to expend for producing eggs." All these 

 are examples of the more general principle that the 

 energy developed in the bodj^ manifests itself in a 

 variety of ways of which the power to procreate is 

 only one. A more special application of the same con- 

 ception as applied to man, and one that has come into 

 prominence through the teaching of a modern school 

 of psychology, is that sexual energy, instead cf being 

 ill-spent or dissipated in unnecessary propagation, 

 may sometimes be more usefully diverted into 

 profitable intellectual channels. 



There are, of course, many exceptions to Spencer's 

 law of the inverse relation between Individuation and 

 Genesis (for example, the fact that wild animals, even 

 when tamed and not confined, will often refuse to 

 breed) ; nevertheless, as a general descriptive statement 

 of the observed facts of nature it represents a true 



conception, at least so far as the higher animals are 

 concerned. But the ways in which conformity to the 

 law is observed are diverse, and in view of the import- 

 ance of the subject, not only theoretically but also 

 practically (in relation to man's control over the 

 fecundity of the domestic animals), it is well worthy 

 of study. 



High Rates of Propagation 



It might at first thought be supposed that an animal's 

 capacity to procreate was controlled mainly by the 

 number of ova or eggs produced by the female, or the 

 number of corresponding reproductive cells (called 

 sperm cells or spermatozoa) given off by the male. 

 In the lower animals this supposition is in a sense 

 correct, for of the total number of eggs produced, say, 

 by a fish, probably the majority are actually spawned, 

 and here the actual fertility is regulated by the number 

 of eggs which become fertilised, and which, escaping 

 the many dangers to which they are subject, are 

 able to complete the process of development. Thus 

 it is stated that the female cod spawns si.x million eggs, 

 of which considerably less than a third are afterwards 

 fertilised. In the higher animals, however, as well as 

 in many of the lower, only a certain proportion of the 

 potential eggs produced by the ovary or female repro- 

 ductive gland ever reach maturity at all, and a still 

 smaller percentage are released from the organ so as to 

 obtain a chance of becoming fertilised and developing 

 into new individuals. The late Professor Francis 

 Maitland Balfour, whose early death biologists have not 

 ceased to deplore, showed long ago that in the higher 

 animals one embryonic egg may develop at the expense 

 of others, and that the eggs which disappear may serve 

 as food material for the one ovum which, owing to a 

 superior vigour or to some chance circumstance relating 

 to its position in the ovary, was able to survive. 



Cannibalism among Eggs 



Thus there is a veritable struggle for existence 

 amongst the reproductive cells during the processes of 

 development within the generative glands, and those 

 cells which survive may do so by taking advantage of 

 the death of others at a very early stage of existence. 

 This fact has been particularly well shown in the 

 freshwater polyp or Hydra, and other species belonging 

 to the same class, for in the developing eggs of these 

 animals the nuclei of other ingested eggs continue to 

 be easily recognisable even after the surviving egg 

 has become fertilised and has begun to develop. 

 Similar observations have been made more recently 

 by Dr. Janet Lane-Claypon on the developing ova of 

 the rabbit, and phenomena of the same kind have been 

 noted in other species of mammals. Dr. Aral has 

 estimated that the female reproductive gland of the rat 



