176 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



relationship. None of them denies 'and many strongly affirm that 

 embryology affords some of the strongest and most convincing evi- 

 dence hi favor of the evolutionary theory. 



Let us examine some of this evidence. To begin with, it should 

 be noted that, hi following out the ontogeny or individual develop- 

 ment, the observer witnesses the formation of something new, not 

 merely the enlargement and unfolding of a pre-existing organism, 

 though the theory of preformation, which was widely accepted in the 

 eighteenth century, looked upon ontogeny precisely in that way, as 

 the growth of a germ which was the miniature of the parent. Such a 

 theory was possible only before the development of microscopic 

 technique had enabled the observer to detect the actual successive 

 steps of change. The egg is a single cell, with the nucleus and all the 

 parts of other undifferentiated cells, though it may be enormously 

 enlarged by the presence of food-yolk. In the hen's egg this food-yolk 

 is quite inert and the activity of development is confined to the minute 

 disc of protoplasm on the outside of the yolk, while in the frog's egg 

 the yolk is disseminated, though not uniformly, throughout the egg 

 and in the mammalian egg, which is microscopic in size, there is no 

 yolk. It is a very remarkable fact that all of the vertebrated animals, 

 fishes, amphibians, reptiles, birds and mammals, however different 

 their habits and modes of life, have a mode of ontogeny which is of 

 even more characteristically and unmistakably the same plan than is 

 the type of their adult structure, which was described in the last 

 chapter. The egg, or the active portion of it, divides in a definite and 

 regular manner into a very large number of cells, which arrange them- 

 selves hi definite layers, an outer and an inner, and within these layers 

 cell-aggregates form incipient organs, which, step by step, take on the 

 adult condition. Not only is the plan and type of development 

 essentially similar throughout the whole phylum of the vertebrates, 

 but, in accordance with the recapitulation theory, many structural 

 features which are permanent in lower forms appear in the embryos of 

 higher and more advanced types. In the latter, however, these 

 features are transitory and, hi the course of development, they either 

 disappear, or are so modified as to be very different, sometimes unrecog- 

 nizable, in the adults. 



At a certain stage of the ontogeny the embryo of a mammal has 

 gill-pouches like a fish, the skeletal supports of the gill-pouches, the 

 arteries and veins which supply them with blood, the structure of the 

 heart, in short, the entire plan of the circulatory system is fish-like. 



