372 



SCIENCE 



[N. S. Vol. XXV. No. 



upon the sexual condition of the gametes 

 in certain of the molds. 



It is only by the further accumulation of 

 facts in various groups of plants and ani- 

 mals that we may at length be in position 

 to determine what if any unifying prin- 

 ciple there may be in this wide-spread phe- 

 nomenon of sexuality. 



• A. F. Blakeslee 



Botanical Laboratoby, 

 Haevakd University 



THE BIOLOGICAL SIGNIFICANCE OF SEXUAL 



DIPPEEENTIATION A ZOOLOGICAL POINT 



OF VIEW 



The line of descent in multicellular ani- 

 mals is through a continuous or discon- 

 tinuous series of sexual generations. In 

 the latter case, there is alternation of gen- 

 erations, either of asexual and sexual or of 

 parthenogenetic and sexual generations. 

 So far as I know, the sexual generation is 

 never absent in the first kind of alterna- 

 tion; there are, however, some partheno- 

 genetic species in which males have never 

 been found, though the structure of the 

 females, or the natural history of the race, 

 proves the former existence of males. 

 There is only one feature common to all 

 forms of sexual reproduction, and that is 

 the union of ovum and spermatozoon to 

 form a single cell, which has the capacity 

 of developing into a new individual of the 

 species. The biological significance of sex 

 must, therefore, lie in the process of fertil- 

 ization; and the interpretation of the fun- 

 damental significance of fertilization must 

 be the answer to our problem. 



Now fertilization is a more general phe- 

 nomenon than sex itself, for it is character- 

 istic of the Protozoa in the form of con- 

 jugation; and it appears to be a growing 

 conviction among students of Protozoa that 

 conjugation is universal in this group. 

 Fertilization brings about biparental in- 



heritance or amphimixis, and some have 

 regarded this as its chief function, in view 

 of the great importance of amphimixis for 

 the process of evolution. But most zool- 

 ogists regard amphimixis as a secondary 

 function of fertilization, and find the chief 

 significance of fertilization in the satisfac- 

 tion of a periodic physiological need of the 

 organism. The ovum usually requires 

 fertilization as a stimulus to development; 

 without it, in most animals, the processes 

 of development either do not begin or soon 

 cease. Observations on normal and arti- 

 ficial parthenogenesis demonstrate that it 

 is not an indispensable requirement for 

 development; however, in most partheno- 

 genetic species fertilization-need arises in 

 certain generations that alternate more or 

 less regularly with the parthenogenetic 

 ones; and those parthenogenetic species in 

 which males are unknown have descended 

 from sexual species, and moreover belong 

 to specialized groups on one side of the 

 main trend of evolution. Among Protozoa 

 there seems to be a periodic need of fertil- 

 ization to maintain the capacity of the 

 species for reproduction. 



"We may then say, with the qualifications 

 already indicated, that among animals at 

 least the law of conjugation is as universal 

 and imperative as the law of hunger. It 

 is thus one of the most general of biological 

 phenomena, with an element of obscurity 

 in it that does not inhere in any other 

 major problem of biology; for, as katab- 

 olism is combustion, the need of hunger 

 to incite the individual to the taking of 

 food is obvious; as the individual survives 

 by adjustment to its environment, it must 

 possess irritability and motility; but why 

 the same food that satisfies for so long fails 

 ultimately to support ebbing vitality among 

 Protozoa, why the line of descent in Meta- 

 zoa should pass through sexual generations 

 — this is the mystery of physiology; and 

 that salvation of the race should reside in 



