248 DARWINISM AND THE PROBLEMS OF LIFE 



their size, only be produced in small quantities ; but on 

 the other hand, the amount of nourishment provided 

 enables the young in them to run through the stages 

 of development more quickly than the free larv^, which 

 have to struggle laboriously for the matter with which 

 to build up their frames. 



When an animal is brought by natural selection to 

 develop in the egg instead of as a free larva, a number 

 of changes will follow. All the characters that can only 

 be of use to a free-living larva are cut out in the 

 embryonic development as so much waste of time and 

 material, and only those ancestral stages will be retained 

 that are necessary for the further development of the 

 specific marks of the animal. Here we come to the 

 essence of the biogenetic law. It is not a law of 

 absolute and universal validity, like the law of gravity, 

 otherwise every animal would have to reproduce exactly 

 in its embryonic development the stages of its ancestry, 

 which is not the case. It is no more than a postulate 

 of the action of natural selection. All the characters of 

 animals must arise in connection with others already 

 existing ; nothing can be developed suddenly and 

 without intermediate stages. Hence if a new organ has 

 been formed from a previous one by gradual modifica- 

 tion, it must follow the same line in its development 

 from the ovum to the adult, because the ontogeny also 

 can only proceed gradually. Thus the biogenetic law 

 demands that just as an animal could only develop 

 further in its ancestral history on the strength of 

 qualities already existing in its predecessors, so in its 

 individual growth it must build gradually on the actual. 



