no AMPHIMIXIS OR ESSENTIAL MEANING OF [XII. 



A question arising out of this view, and one which Hensen 

 doubtfull}^ propounds, is 'whether the "sexual force" could 

 increase to such an extent that males should become super- 

 fluous,' and whether parthenogenesis, like sexual reproduction, 

 could continue, not only for a limited number of generations, but 

 for an unending series. 



As regards an answer to these questions Hensen was quite 

 unbiassed and awaited the decision of facts ; moreover, from 

 his point of view, no theoretical impossibility attended any such 

 increase in the female ' sexual force.' He was, at that time, far 

 nearer to the most recent views on fertilization than those 

 numerous investigators who held parthenogenesis to be the 

 consequence of fertilization which had taken place in earlier 

 generations, and who considered that its effect could never last 

 through an unlimited series of generations, but that the vitalizing 

 or rejuvenating effect of fertilization must be renewed from time 

 to time, or the power of reproduction would be lost. On these 

 fundamental views as to the ' vitalizing of the germ by fertiliza- 

 tion ' depends the reluctance of nearly all writers to recognise 

 the submitted facts of a continuous and purely parthenogenetic 

 reproduction, as for example in the case of the Ostracoda. It 

 is certainly true that absolute proofs of the indefinite duration 

 of this mode of reproduction cannot be obtained ; for unlimited 

 time and innumerable generations are not within the limits of 

 observation ; but who doubts whether the sexual method, with 

 which we are so completely familiar, and which is for this reason 

 spoken of as the usual mode of reproduction, — who doubts 

 whether this can endure without limit ? And yet this assump- 

 tion is as incapable of proof by appeal to experience as the other. 

 It appears to be very difficult to get rid of the deeply rooted idea 

 that fertilization is a vitalizing process, a ' rejuvenescence of life,' 

 although we are quite unable to explain the nature of the renewal 

 which is supposed to take place. The old idea of ' vital force ' 

 unconsciously bears a part in this view, an idea which certainly 

 does not gain any scientific justification because, as Bunge has 

 rightly said, we are to-day very far from laying bare the 

 deepest roots of any one of the processes of life and explaining it 

 by the operation of known forces. I hardly think that we shall 

 ever reach this point, but until the explanation of vital processes 

 by means of the well-known chemical and physical properties 



