FERTILIZATION OF THE OVUM 179 



parable to that of a protozoan race, a long series of cell-divisions being 

 in each case followed by a mixture of protoplasms through conjuga- 

 tion ; and he assumed that, in both cases, conjugation results in reju- 

 venescence through which the energy of growth and division is 

 restored and a new cycle inaugurated. The same view has been 

 advocated by Minot, Engelman, Hensen, and many others. Mau- 

 pas ('88, '89), in his celebrated researches in the conjugation of Infu- 

 soria, attempted to test this conclusion by following out continuously 

 the life-history of various species through the entire cycle of their exist- 

 ence. Though not yet adequately confirmed, and indeed opposed in 

 some particulars by more recent work, 1 these researches have yielded 

 very strong evidence that in these unicellular animals, even under 

 normal conditions, the processes of growth and division sooner or 

 later come to an end, undergoing a process of natural " senescence," 

 which can only be counteracted by conjugation. That fertilization in 

 higher plants and animals does in fact incite division and growth is a 

 matter of undisputed observation. We know, however, that in parthe- 

 nogenesis the egg may develop without fertilization, and we do not 

 know whether the tendency to " senescence " and the need for fer- 

 tilization are primary attributes of living matter. 



The foregoing views may be classed together as the rejuvenescence 

 theory. Parallel to that theory, and not necessarily opposed to or 

 confirmatory of it, is the view that fertilization is in some way con- 

 cerned with the process of variation. Long since suggested by Tre- 

 viranus and more lately developed by Brooks 2 and Weismann 3 is the 

 hypothesis that fertilization is a source of variation a conclusion sug- 

 gested by the experience of practical breeders of plants and animals. 

 Weismann brings forward strong arguments against the rejuvenescence- 

 theory, and regards the need for fertilization as a secondary acquisi- 

 tion, the mixture of protoplasms to which it leads producing variations 

 or rather insuring their "mingling and persistent renewal" 4 

 which form the material on which selection operates. On the other 

 hand, a considerable number of writers, including Darwin, Spencer, 

 O. Hertwig, Hatschek, and others, believe that although crossing may 

 lead to variability within certain limits, its effect in the long run tends 

 to neutralize indefinite variability and thus to hold the species true to 

 the type. 



It is remarkable that we should still remain uncertain as to the physi- 

 ological meaning of a process so general and one that has been the 

 subject of such prolonged research. Both the foregoing general views 

 are in harmony with the results of Darwin's work on variation and 

 with the experience of practical breeders, which have shown that 



1 Cf. Joukowsky, '99. 8 Amphimixis, 1891. 



2 The Law of Heredity, 1883. 4 '99, p. 326. 



