THE CYCLE OF LIFE 377 



the egg-membrane ; the larva is free-hving and able to feed 

 for itseK ; but the larval stage may be suppressed, and 

 then we say that out of the egg-envelope there emerges a 

 young creature. Thus in one type the embryonic develop- 

 ment is succeeded by a long larval period, while in another 

 type the embryonic development eventuates in a young 

 creature Uke a miniature of the parent. 



It is very difficult to discover a quite satisfactory punctu- 

 ation of hfe — ^to say for instance when development stops. 

 As long as the expression of the inheritance goes on, as 

 long as differentiation and integration continue, we may 

 certainly speak of development, but mere increase in size 

 is not development, and it is very difficult to know when 

 to put in the stop. Some would say that there is no stop 

 at aU until death, and that development includes all the 

 normal changes of form and structure that occur through- 

 out hfe, the breaking down in old age being on this view 

 just as much part of development as the building-up of 

 youth. This usage seems more logical than useful, for 

 the changes of senescence are for the most part of the 

 nature of ' involution ' rather than of evolution. 



Others would put in the stop when the hmit of growth 

 is reached, but the brain may go on developing long after 

 that, though in mammals there seems to be no increase 

 in the actual number of brain-cells after birth. Moreover, 

 as we have seen, there are many fishes and reptiles that 

 show no hmit of growth. 



Others would put in the stop when the specific characters 

 are well-defined, when the creature has put on the dress 

 that is its own and no other's. There seems a great deal 

 to be said for this punctuation, but it is open to the ob- 

 jection of excluding much that can be justly called develop- 



