222 ANIMAL PEDIGREES 



yolk with which it is distended, has in the same 

 time made but very slight progress. From this 

 time however other considerations begin to tell. 

 Amphioxus has been able to make this rapid start 

 owing to its relative freedom from food yolk. This 

 freedom now becomes a retarding influence, for the 

 Jarva, containing within itself but a very scanty 

 supply of nutriment, must devote much of its 

 energies to hunting for and to digesting its food, 

 and hence its further development will proceed more 

 slowly. 



The chick embryo on the other hand has an 

 abundant supply of food in the egg itself; it has 

 no occasion to spend time in searching for food, but 

 can devote its whole energies to the further stages 

 of its development. Hence, except in the earliest 

 stages, the chick develops more rapidly than Am- 

 phioxus, and attains its adult form in a much 

 shorter time. 



The tendency of abundant food yolk to lead to 

 shortening or abbreviation of the ancestral history, 

 and even to the entire omission of important stages, 

 is well known. The embryo of forms well provided 

 with yolk takes short cuts in its development, 

 jumps from branch to branch of its genealogical 

 tree, instead of climbing steadily upwards. 



An excellent illustration of the influence of food 

 yolk on development is afforded by the life histories 

 of frogs. The common frog, Rana temporaria, lays 

 as is well known eggs of small size, about ^ inch 

 in diameter. A small egg can only contain a 

 limited amount of food yolk, and hence the young 



