l2!4 CIKCUMSTANCBS FAVOURABLE TO THE [Chap. IV. 



rents of water offer an obvious means for an occasional 

 cross. As in the case of flowers, I have as yet failed, 

 after consultation with one of the highest authorities, 

 namely, Professor Huxley, to discover a single hermaph- 

 rodite animal with the organs of reproduction so per- 

 fectly enclosed that access from without, and the oc- 

 casional influence of a distinct individual, can be shown 

 to be physically impossible. Cirripedes long appeared 

 to me to present, under this point of view, a case of 

 great difficulty; but I have been enabled, by a fortunate 

 chance, to prove that two individuals, though both are 

 self-fertilising hermaphrodites, do sometimes cross. 



It must have struck most naturalists as a strange 

 anomaly that, both with animals and plants, some spe- 

 cies of the same family and even of the same genus, 

 though agreeing closely with each other in their whole 

 organisation are hermaphrodites, and some unisexual. 

 But if, in fact, all hermaphrodites do occasionally inter- 

 cross, the difference between them and unisexual species" 

 is, as far as function is concerned, very small. 



From these several considerations and from the many 

 special facts which I have collected, but which I am 

 unable here to give, it appears that with animals and 

 plants an occasional intercross between distinct indi- 

 viduals is a very general, if not universal, law of nature. 



Circumstances favourable for the production of new forms 

 through Natural Selection. 



This is an extremely intricate subject. .A great 

 amount of variability, under which term individual dif- 

 ferences are always included, will evidently be favour- 

 able. A large number of individuals, by giving a better 

 chance within any given period for the appearance of 



