IN THE THEORY OF NATURAL SELECTION. 325 



also nature has met the difficulty by another adaptation. As I 

 have shown in a previous paper l , the heterogeny of the species of 

 Daphnidae which inhabit such pools is modified in such a manner, 

 that only the first generation produced from the resting 1 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 Dapknidae, heterogeny may 

 pass into pure parthenogenesis by the non-appearance of the sexual 

 generations. This seems to have taken place in certain species of 

 osmina and Ckydoru*, 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 produced in a similar manner, by the 

 non-appearance of males. 



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

 genesis is by no means so clear in all cases. Sometimes, 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 modes of reproduction 

 is in itself a sufficient proof of the validity of the conclusions which 

 have just been advanced. 



1 Weismann, ' Naturgeschichte der Daphnoiden,' Zeitschrift f. wiss. Zool. XXIII. 

 1879. 



