220 ANIMAL PEDIGREES 



one, but an exceedingly difficult one to draw in 

 practice. The causes which prevent development 

 from being a strict recapitulation of ancestral 

 characters, the modes in which these came about, 

 and the influence which they respectively exert, 

 are matters which are greatly exercising embryo- 

 logists ; and the attempt to determine them has as 

 yet met with only partial success. 



The most potent and the most widely spread of 

 these disturbing causes arise from the necessity of 

 supplying the embryo with nutriment. This acts 

 in two ways. If the amount of nutritive matter 

 with 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 selection may act so as to introduce modi- 

 fications of the ancestral history, spurious additions 

 to the text. 



If on the other hand the egg contain within 

 itself a considerable quantity of nutrient matter, 

 then the period of hatching can be postponed 

 until this nutrient matter has been used up. The 

 consequence is that the embryo hatches at a much 

 later stage of its development, and if the amount of 

 food material is sufficient may even leave the egg 

 in the form of the parent. In such cases the earlier 

 developmental phases are often greatly condensed 

 and abbreviated ; and as the embryo does not 

 lead a free existence, and has no need to exert it- 

 self to obtain food, it commonly happens that these 

 stages are passed through in a very modified form, 



