D.— ZOOLOGY 91 



itself. In general this latter or intra- specific type of selection is more 

 widespread than the inter-specific. 



It is a common fallacy to think of natural selection as first and foremost 

 a direct struggle with adverse weather, with enemies or with the elusive 

 qualities of prey. The most important feature of the struggle for 

 existence is the competition of members of the same species for the means 

 of subsistence and for reproduction. Surprise has been expressed by 

 some biologists at the fact that in New Zealand, domestic pigs which 

 have become feral have, in spite of the absence of predatory enemies, 

 reverted to something like the wild type ; but in competition for food 

 and reproduction the leaner and more active wild type must clearly 

 have a strong relative advantage over the fatter and more sluggish domestic 

 forms. 



It is another fallacy to imagine that because the major elimination 

 of individuals occurs in one period of life, therefore selection cannot 

 act with any intensity on the phase of minimum numbers. It has, 

 for instance, been argued that because the main elimination of butter- 

 flies takes place during the larval stage, therefore elimination of the 

 imagines by birds or other enemies can have no appreciable selective 

 effect, and that therefore any protective or warning or mimetic colouring 

 which they exhibit cannot have any adaptive significance. However, 

 it is the adults which reproduce, and a one per cent, advantage of one 

 adult type over another will have precisely the same selective effect 

 whether the adults represent ten, one, or one-tenth of one per cent, of 

 the number of fertilised eggs produced. The same applies to those plants 

 in which the main elimination occurs during the seedling stage. Selection, 

 in fact, can and does operate equally effectively at any stage of the life- 

 cycle. Further, elimination is far from being the only tool with which 

 selection operates. Differential fertility of the survivors is also important, 

 and in man and many plants is probably the more influential. 



Rate- Genes and Selection. 



But, as Haldane has stressed, the results of selection at one period of 

 the life- cycle may have repercussions on other periods and affect the 

 species as a whole in unexpected ways. Perhaps the best example which 

 he gives is that of intra-uterine selection in polytocous mammals . Here there 

 must be intense competition, since a considerable percentage of every 

 litter dies in utero and rapidity of growth must be at a premium. Haldane 

 suggests with some plausibility that any rapidity of pre-natal growth 

 thus acquired is likely to be transferred in whole or in part to post-natal 

 life as well, and that intra-uterine selection may thus help to account for 

 the progressive increase in size seen in so many mammalian lines during 

 their evolution. At any rate, the converse seems to hold, namely that 

 on account of intra-uterine selection it would be impossible for a poly- 

 tocous mammal to slow down its rate of development. One of the most 

 characteristic features of man is precisely such a slowing down of general 

 rate of development. Without it he could not in all probability have 

 become fully human or biologically dominant. This condition could not 

 have occurred in a polytocous form. It was only after man's ancestors 



