334 SIGNIFICANCE OF SEXUAL REPRODUCTION [V. 



would hardly be able to exist in small pools filled by the rain ; 

 but here also nature has met the difficulty by another adapta- 

 tion. As I have shown in a previous paper ', the heterogeny 

 of the species q{ Daphnidae which inhabit such pools is modi- 

 fied in such a manner, that only the first generation pro- 

 duced from the resting eggs consists of purely parthenogenetic 

 females, while the second includes many sexual animals, so 

 that resting eggs are produced and laid, and the continuance 

 of the colony is secured a few days after it has been first 

 founded ; viz. after the appearance of the first generation. 



But it is also certain that in the Daphnidae, heterogeny may 

 pass into pure parthenogenesis by the non-appearance of the 

 sexual generations. This seems to have taken place in certain 

 species of Bosmina and Chydorus, although perhaps only in 

 those colonies of which the continuance is secured for the 

 whole year ; viz. those which inhabit lakes, water-pipes, or 

 wells in which the water cannot freeze. In certain insects also 

 (e. g. Rhodites rosae) pure parthenogenesis seems to be pro- 

 duced in a similar manner, by the non-appearance of males. 



But the utility which we may look upon as the cause of 

 parthenogenesis is by no means so clear in all cases. Some- 

 times, especially in certain species of Ostracoda, its appearance 

 seems almost like a mere caprice of nature. In this group of 

 the Crustacea, one species may be purely parthenogenetic, 

 while a second reproduces itself by the sexual method, and 

 a third by an alternation of the two methods : and yet all these 

 species may be very closely allied and may frequently live in 

 the same locality and apparently with the same habit of life. 

 But it must not be forgotten that it is only with the greatest 

 difficulty that we can acquire knowledge about the details of 

 the life of these minute forms, and that where we can only 

 recognize the appearance of identical conditions, there may be 

 highly important differences in nutrition, habits, enemies and 

 the means by which they are resisted, and in the mode by 

 which the prey is captured— circumstances which may place 

 two species living in the same locality upon an entirely different 

 basis of existence. It is not merely probable that this is the 

 case ; for the fact that certain species have modified their 



^ Weismann. ' Naturgeschichte der Daphnoiden,' Zeitschrift f. wiss. 

 Zool. XXIII. i'879. 



