264 OUTLINES OF EVOLUTIONARY BIOLOGY 



to another, as, for example, at a later stage, from the chrysalis to 

 the butterfly, the development really progresses quite slowly 

 and gradually internally, although the external appearance may 

 for a long time remain unaltered and give the impression that 

 it is completely at a standstill. 



The fact that the young organism cannot commence its life at 

 the stage reached by its parents, but has to make a fresh start 

 from the beginning and go through a whole series of stages 



FIG. 117. A. Chick Embryo of about 5| days, X 5. B. Babbit Embryo of 

 about 13 days, X 5. In both cases the foetal membranes and yolk-sac 

 have been removed. (From photographs.) 



before reaching the adult condition, is very significant from the 

 point of view of the evolutionist. Still more significant is the 

 fact that different organisms all commence at the same stage 

 as unicellular eggs and come to diverge further and further 

 from one another in structure as their development progresses. 

 If we examine a number of series of embryos belonging to 

 different vertebrate types, no matter how widely the adult 

 animals may differ from one another, we shall find that as we 

 trace their life-histories backwards they gradually converge until, 

 while still at a relatively advanced stage of development, the 

 different embryos come to resemble one another so closely that 



