OPTIMUM CONDITIONS 127 



distinction has to be made between direct and 

 indirect development, so in the intra - uterine 

 development of Peripatus a similar contrast 

 occurs. What is meant by a normal course of 

 development is one that is not disturbed by 

 adventitious factors. We must assume certain 

 optimum conditions which need by no means 

 be the most primitive and indeed on a priori 

 considerations are not likely to have been the 

 earliest conditions encountered by the amphibious 

 palaeozoic prototracheate ancestors of our terres- 

 trial arthropods. Optimum conditions may be 

 brought about by the acquisition of a little yolk 

 in the egg or by the loss of a little yolk ; and 

 this may make a considerable difference in the 

 behaviour of the embryos or larvae, as the case 

 may be. 



All methods of development may be regarded 

 as more or less cenogenetic, and the selection of 

 a particular type as primitive may be purely 

 arbitrary ; but there can hardly be two opinions 

 as to a normal or direct course which can be 

 established as a legitimate standard or constant. 

 Any life-history can recapitulate certain ancestral 

 or palingenetic features, some more recent, others 

 more remote. Different species may provide 

 seemingly identical conditions for the maturation 

 of the ova and the development of the young ; 

 but it does not follow that the reactions or adap- 

 tations to these conditions will be identical. 



