' ' E VOL UTION " AND EPIGENESIS. 47 



a marvellous bud-like miniature of the adult, it included the next gener- 

 ation, and the next, and the next, and all future generations. Germ lay 

 within germ, preformed in transparency, and in successively smaller 

 miniature, after the fashion of an infinite juggler's box. We laugh at 

 this, but we need not laugh too much, for the preformationists, though 

 entirely wrong and crude in their facts, were right in two of their ideas, 

 • — that the germ, in reality, does contain the possibility of a future 

 organism, and that it has relations, not only to the animal into which it 

 develops, but also to generations following. 



In the middle of the seventeenth century, however, Harvey had reached 

 concktsions which might have saved much blundering. Studying the 

 development of the chick, — as Greek naturaUsts had tried to do well-nigh 

 two thousand years before, as we are doing still in our embryological 

 laboratories, — Harvey maintained that every animal was produced from 

 an ovum {ovum esse primordium commune omnibus aniinalibus), and 

 that organs arose by new formation (epigenesis), not by the mere expan- 

 sion or "evolution" of some invisible preformation. 



But the great champion of epigenesis was Caspar ' Friedrich Wolff, 

 who, in his doctorial dissertation of 1759, traced the chick back to a 

 layer of organised particles (the familiar cells of to-day), in which there 

 was no likeness of the future embryo, far less adult. 



Wolff was long in finding successors, but in 1824 Prevost and Dumas 

 described the division of the ovum into masses (segmentation) ; in 1827 

 Von Baer discovered the mammalian ovum ; while Wagner, Von Siebold, 

 and others elucidated the real nature of the spermatozoa. 



A great step was made in 1838-9, when Schwann and Schleiden 

 formulated the "cell theory," according to which every organism is 

 made up of cells, and starts from a cell. From this date modern 

 embryology began, and has of late years made advances greater perhaps 

 than those of any other department of zoology. 



Sexual Reproduction. — There is apt to be a lack of clear- 

 ness in regard to sexual reproduction, because the process 

 which we describe by that phrase is a complex result of 

 evolution. It involves two distinct facts : — (a) the liberation 

 of special germ-cells from which new individuals arise ; (V) 

 the occurrence of two different kinds of germ-cells— ova and 

 spermatozoa, which are fertile only when they unite (ferti- 

 lisation). Furthermore, these dimorphic reproductive cells 

 are produced by two different kinds of individuals (females 

 and males), or from different organs of one individual, or at 

 different times within the same organ (hermaphroditism). 



It is quite evident that organisms might have gone on 

 multiplying asexually, by detaching overgrown portions of 

 themselves which had sufficient vitality to develop mto 

 complete forms. But a more economical method is the 

 liberation of special germ-cells, in which the qualities of the 



