OTHER THEORIES OF EVOLUTION 329 



common tendency in evolution for development to be accel- 

 erated (i.e. for certain characters to appear earlier in ontogeny), 

 or to become retarded, he suggests that this is due to the times 

 of action of certain genes being pushed forward or back in the 

 course of development. He points out (p. 21) that accelera- 

 tion and retardation are probably influenced by two types 

 of selection. In animals which produce many young (e.g. 

 rodents) there will be a certain measure of prenatal competi- 

 tion, and rapid growth will be of great selective value, and 

 the slower-growing individuals will be weeded out. ' There 

 will be a tendency to cut short the period of intense competition 

 and push back the first appearance of characters as early as 

 possible. Conversely, in forms in which ' a larva or embryo is 

 well suited to its surroundings and can go on growing in rela- 

 tively slight danger there will be a tendency to prolong the 

 embryonic phase.' In such forms we may expect retardation. 



It should be possible to check this ingenious hypothesis. 

 If it is correct, we ought to find accelerated development in 

 forms with numerous embryos and retarded development in 

 those with few embryos. Haldane (1932, p. 124) cites the 

 retarded development of man as an instance of the latter. 



Until Haldane's hypothesis is thoroughly tested on the 

 lines suggested above, it is impossible to do more than suspend 

 judgment as to its value. It is a little difficult to see how 

 it applies to (e.g.) the extinct forms of Brachiopods and 

 Ammonites, in which in all probability development took 

 place outside the maternal body. We suspect that many 

 tachygenetic phenomena take place in forms in which there is 

 no such competition as Haldane describes. 



Castle (1932, p. 365) points out that, though Haldane 

 had in mind rapidity of differentiation rather than of growth 

 in size, the principle will apply with equal force to increase 

 of size, both in pre-natal and post-natal competition. He 

 instances his own very significant observation that, when 

 ' large race ' and ' small race ' rabbits are put to a common 

 foster-mother, the former push the smaller young away and 

 monopolise the milk-supply. 



(b) Attempts have been made to explain evolutionary 

 trends which exhibit stages resembling the youthful, mature 

 and senescent phases of individual ontogeny, in terms of pro- 

 gressive physiological changes. Racial senescence is regarded 



