98 THE DARWINIAN THEORY 



by later hands, and at times so cunningly as to defy 

 detection. 



The chief disturbing cause arises from the necessity 

 of supplying the embryo with nutriment. This acts 

 in two ways. If the amount of nutritive material in 

 the egg is small, then the young animal must hatch 

 early, and in a condition in which it is able to obtain 

 food for itself. In such cases there is of necessity 

 a long period of larval life during which natural selec- 

 tion may act so as to introduce modifications of the 

 ancestral history, or spurious additions to the text. 

 If, on the other hand, the egg contains a considerable 

 quantity of nutrient matter, then the period of hatch- 

 ing can be postponed until this has been used up. 

 The consequence is that the embyro hatches at a 

 much later stage in its development, and if the 

 amount of food material, or food yolk as it is called, 

 is enough, may even leave the egg in the parent 

 form. 



This varying condition, as regards amount of 

 food-yolk, affects recapitulation — i.e., the tendency of 

 ^the embryo to pass through the ancestral stages — in 

 two principal directions. If there is very much food- 

 yolk, there is a tendency for the embryo to shorten 

 its development by the omission of certain of the 

 ancestral stages, and especially by the suppression 

 of characters which, though functional in the 

 ancestors, are of no use in the adult state of the 

 animal itself. Thus tadpoles, after hatching, breathe 

 for a time by gills : this gill-breathing condition being 

 an ancestral one for all Vertebrates. In the West 

 Indies there is a little frog (Hy lodes) which lays its 



