DISPERSAL OF INFUSORIA. 289 



tion. Its principal propositions are as follows : The struggle for 

 existence and the selection it gives rise to cannot by itself lead 

 to the formation of new species ; this can only take place when, 

 in the first place, one or a few specimens of a species are intro- 

 duced at a variable stage into a new home ; when, secondly, 

 these by their removal to a distance are prevented from cross- 

 breeding with typical individuals of the parent stoct ; and when, 

 thirdly, a selection is effected by the external conditions of 

 existence among the varieties arising iii the new colony-^'" 



Setting aside, for the moment, the question as to whether 

 this theory is really opposed to Darwin's or no, we will in the 

 first place inquire whether in its strict application it is actually 

 capable of explaining all the facts occurring in nature. And I 

 must at once confess that in this respect I can by no means 

 admit "Wagner to be right. 



Many kinds of Infusoria are distributed over the whole 

 globe in sharply defined species. Now it is quite certain that 

 among these creatures no separation by removal exists to pre- 

 vent the free interbreeding of any variety with the parent 

 form. They are most easily transportable, either living in 

 water, or when dry, as dust by the wind, and indeed we see 

 that in no other animal group are there species so cosmopolitan 

 and so universally distributed as among the Infusoria ; never- 

 theless the characters of the different species remain very 

 constant. Between the tropics there is no sharply demarcated 

 breeding period for the greater number of marine animals, so 

 that fully grown individuals, young ones, and eggs, can at all 

 times be found side by side in every stage of development. 

 Moreover the fertilisation of the eggs is efiected while they 

 are freely swimming in the ocean in the case of almost all 

 Echinodermata, all Coslenterata, many worms, most bivalves, 

 and many Tunicata, Brachiopoda, and Bryozoa ; and in the few 

 viviparous forms of these groups no union of the sexes takes 

 place. On the contrary, the fertilisation of the ova is left to 

 chance ; the spermatozoa or male element being expelled into 

 the sea and conveyed by currents which bring them into contact 

 with the eggs or with the female parent organism. In these 

 and also in all those marine species which have free-swimming 



