I4 8 THE EVOLUTION OF SEX. 



a favorite hypothesis, and imagination supplied more than modern 

 magnifiers to those observers who detected in the spermatozoon the 

 members and lineaments of the future organism. After this the dis- 

 covery that the sperm supplies half the nucleus of the fertilized ovum, 

 and half the nuclei of the two first daughter-cells, seems almost a little 

 thing. The polemic of modern science has this advantage at least, 

 that when two competent authorities on the same subject assert the 

 same thing, we may generally believe them. 



(r) The third opinion, that both elements are of essential and 

 inseparable import, is obviously alone consistent with the facts. This 

 view also has had its gradual development, only one phase of which 

 need be noticed. Even after the nature of the spermatozoa as male- 

 cells was recognized, that is to say even within the last fifty years, an 

 old conception of the male influence lingered persistently. This, 

 namely, that contact was not essential, but that a " sort of conta- 

 gion," a " breath or miasma," " a plastical vertue," " without, touch- 

 ing at all, unless through the sides of many mediums," was sufficient 

 to effect what we call fertilization. The above expressions are used 

 by Harvey, who further says, " this is agreed upon by universal con- 

 sent, that all animals whatever, which arise from male and female, are 

 generated by the coition of both sexes, and so begotten as it were per 

 contagium a/iquod." De Graaf attempted in vain to give more pre- 

 cision to this "contagion" in his theory of an "aura seminalzs," 

 or seminal breath which passed from the male fluid to the ovum. But 

 the conception of an ' • aura ' ' was only a verbal cloak for that absence 

 of definite knowledge which the slow progress of observation still 

 necessitated. The theory was partly strengthened by a number of 

 erroneous observations, which seemed to show that successful fertiliza- 

 tion could occur when the genital passages of the female were appar- 

 ently blocked by malformation or disease. Spallanzani gave a death- 

 blow to the theory of an "aura," by showing experimentally that con- 

 tact of the male fluid with the ovum was absolutely necessary. Even 

 he, however, went away from the true conclusion, by maintaining that 

 the fertile male fluid of toads was destitute of spermatozoa. That 

 the above vague conceptions have been replaced by the certain con- 

 clusion that intimate cellular union is the sine qua non of fertiliza- 

 tion, we have already emphasized. 



II. Modern Theories of Fertilization. — Morphological. — 



Recent investigators of the facts of fertilization have generalized their 

 results in different ways according to their dominant bias. Some 

 mainly restrict themselves to stating the morphological facts, and to 

 emphasizing the relative importance of cell-substance and of nuclei in 



