REPRODUCTION. 881 



the species. From his observations upon the Infusoria, Maupas 1 has brought 

 forward valuable evidence which has been quoted in favor of this view. Sty- 

 lonychia normally produces by fission 130 to 180 generations or individuals, 

 Onyuhodromus 140 to 230, and Leucophrys patula 300 to 450, after which con- 

 jugation is necessary to continued division. If conjugation be prevented, the 

 individuals become small, their physiological powers become weakened, their 

 nuclei atrophy, and the chromatin disappears ; all of which changes are evidence 

 of the oncoming of senile degeneration, and this ultimately results in death. 

 Analogous to this is doubtless the fact, pointed out by Hertwig, 2 that in sexual 

 animals an unfertilized ovum within the oviduct soon becomes over-mature 

 and enfeebled, and subsequent fertilization, even though possible, is abnormal. 

 Even if the idea of " rejuvenescence" be regarded as fanciful and as a com- 

 parison rather than an explanation, it seems to be a principle of nature that 

 occasional fusion of one line of descent with another is necessary to continued 

 reproduction and continued life. 



A second theory, defended by Hatschek and Hertwig, argues that sexual 

 reproduction prevents variation, and thus preserves the uniformity of the race. 

 The mingling of two different individuals possessing different qualities must 

 give rise to an individual intermediate between the parents, but differing from 

 them. Such differences between parents and offspring are numerous, but in a 

 single generation are minute, and they are easily obliterated by a subsequent 

 union, which latter in turn gives rise to other minute differences. Hence sexual 

 reproduction, although constantly producing variations, as constantly eradicates 

 them, and, by striving always toward the mean between two extremes, tends 

 toward homogeneity of the species. The essential truth of such a view seems 

 obvious. 



A third theory, advocated by Weismann and Brooks, is quite the opposite 

 of the last, and maintains that the meaning of sexual reproduction lies in the 

 production of variations. " The process furnishes an inexhaustible supply of 

 fresh combinations of individual variations." These minute variations, seized 

 upon by natural selection, are augmented and made serviceable, and a variety, 

 better able to cope with the conditions of existence, results. The transformation, 

 not the homogeneity, of the species is thereby assured. The two latter views are 

 not necessarily mutually exclusive. Both claim that fertilization brings into 

 evidence variations. It is quite conceivable that subsequent fertilizations may 

 obliterate some and augment others, the result of union being the algebraic sum 

 of the characteristics contributed by the two sexes. 



Primary and Secondary Characters. In the human species, as in all 

 the higher sexual animals, the characters of sex, anatomical, physiological, and 

 psychological, are divisible into two classes, called primary and secondary. 

 Primary sexual characters are those that pertain to the sexual organs them- 

 selves and to their functions. They are naturally the most pronounced of all 



1 E. Maupas : Archives de Zoologie experimentale et generate, 2e srie, vii., 1889. 



2 O. und E. Hertwig : Experimentelle Studien am thierischen Ei vor, wdhrend und nach der 

 Eefruchtung, i., 1890. 



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