THE RECAPITULATION THEORY. 29 



history, the modes in which these came about, and the influence 

 which they respectively exert, are problems which are as yet only 

 partially solved. 



Of these disturbing causes, the most potent and the most 

 widely spread arises from the necessity of supplying the embryo 

 with nutriment. This acts in two ways. 



If the amount of nutritive matter within the egg be small, 

 then, as we have already seen, the young animal must hatch 

 early and in a very imperfectly developed condition. In such 

 cases, as in Amphioxus or the frog, there is of necessity a long 

 period of larval life, during which natural selection may act so 

 as to introduce modifications of the ancestral history, spurious 

 additions to the text. Of such ' larval organs,' the long spines 

 that form conspicuous features in the young, free swimming 

 larvae of sea urchins, or of crabs, are good examples. 



If, on the other hand, the egg contain within itself a con- 

 siderable quantity of nutrient matter, then the period of hatch- 

 ing 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, as in the case of the chick, leave 

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

 developmental phases are often greatly condensed and abbre- 

 viated ; and as the embryo does not lead a free existence, and has 

 no need to exert itself to obtain food, it commonly happens that 

 these stages are passed through in a very modified form, the 

 embryo being, as in a three-day chick, in a condition in which 

 it is clearly incapable of independent existence. 



The effect of a greater or less amount of food-yolk on the 

 recapitulation of ancestral characters has been summed up by 

 Balfour thus : ' There is a greater chance of the ancestral 

 history being lost in forms which develop in the egg, and of its 

 being masked in those which are hatched as larvee.' 



There are a number of other causes, besides food-yolk, which 

 tend to modify the ancestral history as preserved in individual 

 development. The following list gives a brief summary of the 

 more important of these. 



