CHAP. XXII. TRANSMUTATION. 447 



Secondly, the same powers of flight which enabled the first 

 bats to reach Madeira or the Canaries would bring others 

 from time to time from the African continent, which, mixing 

 with the first emigrants and crossing with them, would check" 

 the formation of new races, or keep them true to the old 

 types, as is found to be actually the case with the birds of 

 Madeira and the Bermudas. 



This would happen the more surely if, as Mr. Darwin has 

 endeavored to prove, the offspring of races slightly varying 

 are usually more vigorous than the progeny of parents of 

 the same race, and would be more prolific, therefore, than the 

 insular stock which had been for a long time breeding in 

 and in. 



The same cause would tend in a still more decided manner 

 to prevent the seals from diverging into new races or " inci- 

 pient species," because they range freely over the wide ocean, 

 and may therefore have continual intercourse with all other 

 individuals of their species. 



Thirdly, as to peculiar species, and even genera, of bats 

 in islands, we are j)erhaps too little acquainted at pi'esent 

 with all the species and genera of the neighboring continents 

 to be able to afiirm, with any degree of confidence, that the 

 forms supposed to be peculiar do not exist elsewhere, — those 

 of the Canaries in Africa, for example. But, what is still 

 more important, we must bear in mind how many species^ 

 and genera of post-pliocene mammalia have everywhere 

 become extinct by causes indejDcndent of Man. It is always 

 possible, therefore, that some types of cheiroptera, originally 

 derived from the main land, have survived in islands, although 

 they have gradually died out on the continents from whence 

 they came ; so that it would be rash to infer that there has 

 been time for the creation, whether by variation or other 

 agency, of new species or genera in the islands in question. 



As to the rodents and cheiroptera of Australia, we are as 



