2l6 AMPHIMIXIS OR ESSENTIAL MEANING OF [XII. 



solve —namely, the remarkable limitation of parthenogenesis to 

 a few definite groups. It is only found in Crustacea, Insecta, and 

 Rotifers, and not among Vermes, Coelenterates, and Echino- 

 derms-^ : furthermore, it does not exist in the two higher groups 

 of JMolluscs and Vertebrates. The solution to the problem is 

 found in the suggestion that the lower groups of animals 

 dispense with parthenogenesis, because it is unnecessary to 

 them. Whenever increase without amphimixis became advan- 

 tageous, it was more readily and better supplied by fission and 

 budding. The absence of parthenogenesis in the higher groups 

 of animals may probably be explained on the supposition that no 

 force has appeared which would render it advantageous for 

 amphimixis to be separated from the existing method of in- 

 crease. This is especially clear when we investigate the 

 grounds on which it must have become advantageous among 

 the Arthropods. 



The periodical occurrence of unfavourable conditions of life 

 has often been suggested as the cause of the appearance of 

 parthenogenesis in Arthropods and Rotifers. I need only refer 

 to my already quoted work on Z)<^///;^/6/ir7^, in which this question 

 is considered at length. Whenever a species lives scattered over 

 a small area subject to rapidly changing external conditions 

 which are, for a short time, favourable to life and multiplication, 

 and then suddenly become unfavourable or even destructive, — 

 it must be a great advantage for the increase of individuals to take 

 place with the greatest possible rapidity during the favourable 

 periods. As indicated in my former work, the advantage of 

 parthenogenesis in such cases lies in the fact that multiplication 

 must become many times more effective when every individual 

 is a female, or, to express the thought in more general terms, 

 when every single germ-cell can produce a new animal. A 

 further acceleration ensues from the omission of that retarda- 

 tion of development which is implied by the occurrence of 

 copulation and fertilization. 



From this point of view we can not only explain the appear- 

 ance of parthenogenesis in general, but also its special form in 



^ I am aw^are that it is believed to occur in some Coelenterates, but it 

 seems to me doubtful whether any true parthenogenesis takes place. 

 And, in any case, isolated exceptions do not invalidate the significance 

 of the rule. 



