82 THE EVOLUTION OF SEX. 



§ I. The Ovum TIieo?y. — It is now a commonplace of 

 observation and established fact, that all organisms, reproduced 

 in the ordinary way, start in life as single cells. We see insects 

 laying their ova upon plants, or fishes shedding them in the 

 water, and may watch how these cells, provided they be 

 fertilised, give rise eventually to the adult organisms. Con- 

 veniently in the ordinary frog-spawn from the ditch, we can 

 read, what was for so long a riddle, how development proceeds 

 by successive cell-divisions and by arrangement of the multiple 

 results. Readily seen in many instances, it is true of all cases 

 of ordinary sexual reproduction, that the organism starts from 

 the union of two sex-cells, or that it is with the division of 

 a fertilised ovum that development begins. 



This profound fact, technically known as the "ovum 

 theory,"' has been not unjustly called by Agassiz "the greatest 

 discovery in the natural sciences of modern times." We shall 

 the better realise the magnitude of the difference which its 

 recognition has introduced into biology, if we briefly review 

 the history. 



§ 2. The History of Tnibryology, Evolution a?id Epigenesis. 

 — The development of the chick, so much studied in embryo- 

 logical laboratories to-day, was the subject of inquiry two thou- 

 sand years ago in Greece. Some of the conspicuous marvels of 

 reproduction and development were persistently but fruitlessly 

 speculated about throughout centuries. It was only during 

 the scientific renaissance of the seventeenth century that the 

 inquiry became more keen and sanguine, and began to rely to 

 some extent at least on genuine observation. 



id) Harvey (165 1), with the aid of magnifying glasses 

 (^perspecillcE\ demonstrated in the fowl's Q.g<g the connection 

 between the cicat7'icula of the yolk and the rudiments of 

 the chick, and also observed some of the stages of uterine 

 life in mammals. ]\Iore important, however, were his far- 

 sighted general conclusions, — (i.) That every animal was pro- 

 duced from an ovum {ovum esse primo7'dium comimme omnibus 

 a?iijnalibus); and (2.) That the organs arose by new formation 

 {epigenesis), not from the mere expansion of some invisible pre- 

 formation. In this generalisation, without however any abandon- 

 ment of the hypothesis of spontaneous generation of germs, he 

 strove, as he said, to follow his master Aristotle, and was in so 

 doing as far ahead of his contemporaries as a strong genius 

 usually is. Before Harvey, the observational method had 



