148 THE POPULAR SCIENCE MONTHLY. 



snows of thousands of winters, receiving new additions at its upper 

 end, and at the same time melting away at its lower, is no bad repre- 

 sentation of the long series of hereditary features, once variations, 

 which form so large a part of every organism. If the glacier were not 

 in motion, but stationary, so that the melting of the oldest portion 

 and the additions to its upper end should gradually carry the body of 

 ice up to higher and higher levels, we should have a very perfect paral- 

 lel to the evolution of an organism by variation and heredity. 



The steps in this progress are embodied in a long series of individ- 

 uals, each of which is, either immediately or indirectly, the product 

 of a fertilized egg or seed, through which the laws of heredity and va- 

 riation act, to bind the separate individuals into a progressive whole. 

 The seeds and eggs with which we are most familiar are highly com- 

 plicated, and consist of the protoplasmic germ, which is intimately 

 united to a mass of food destined to be converted into protoplasm dur- 

 ing development. 



The germ with its food forms the yolk of such an egg as that of 

 the bird, and is surrounded by layers of albumen, which are also used 

 as food, and by a complicated series of investing membranes. It ori- 

 ginates in a special organ, the ovary, and is incapable of perfect devel- 

 opment until it has been fertilized by the male reproductive element. 

 In its earliest stage of growth it is simply one of the cells or histo- 

 logical elements of the ovary, but as it grows it soon becomes very 

 much larger than an ordinary cell, and its protoplasm becomes filled 

 with food material, and the outer layers and walls are added to it. 

 In many animals, the external envelopes are wanting, and the egg is 

 simply a very large ovarian cell, filled with food material, and capable 

 of developing, under the influence of the male element, into a new or- 

 ganism. In still other animals the food-yolk is wanting, and the egg is 

 small, and does not differ from an ovarian cell ; and in still other ani- 

 mals the ovaries are lacking, and cells may become sj)ecialized as ova 

 in various parts of the body. 



The series is so complete that we may be certain that we are com- 

 paring strictly homologous structures, and we may therefore conclude 

 that the egg is nothing but one of the cells of the body, which may, 

 when acted upon by the male element, develop into a neM organism, 

 substantially like its parents, with some of the individual peculiarities 

 of each of them, and also with new peculiarities of its own. 



From the necessity for impregnation in most cases, it has been 

 assumed that the essential function of the male element is to quicken 

 the germ, and thus start the process of development. It is true that it 

 does have this function in many cases ; but comparative study shows 

 that the egg itself is alive, and does not need quickening, and that 

 this must be regarded as a secondary and derived function of the male 

 element, not the essential and primitive function. 



That this is the case is shown by the fact that, while the earlier 



