DISPERSAL OF SPECIES. 17 



One of the great delights in dealing with Mr. Darwin is the perfect fairness with which he 

 states every point connected with his case. The unconscious bias, from which few men escape, seems 

 to be wanting in his constitution. He alwaj's tells the facts which make against him as fully and 

 fairly as those which are in his fovour. And while we may feel unable at the moment to make head 

 against the current of ingenious suggestions, illustrations, and possibilities, with which he hurries us 

 along, he always gives us the real facts to anchor by, so that we can, when we think joroper, pull up 

 and form our own judgment. Now, on this topic he has given us some remarkable instances 

 opposed to his hypothesis. He mentions various islands only separated from others or the main- 

 land by a narrow channel where the species are wholly different. He refers to the Straits of 

 Macassar as separating two widely distinct Mammalian Faimas. Such a separation was first sug- 

 gested by MuUer, and is laid down on his authority in Berghaus' "Physical Atlas."* He (Miiller) 

 ran the line of division up the east side of Celebes, whereas Mr. Earl and Mr. Wallace (more 

 particularly the latter) have since shown that it lies to the west of that island ; and instead of cutting 

 the Island of Timor in two, as was supposed by Miilkr to be the case, passes up the narrow straits 

 (only a few miles wide), between the islands of Bali and Lombock. Mr. Wallace also notices 

 that " Java possesses numerous birds which never pass over to Sumatra, though thej^ are separated 

 by a strait only fifteen miles wide, and with islands in mid-channel." t Of course this can 

 only apply to non-migratorial birds, and I have already suggested a cause why even birds may 

 often be kept within the limits of their original bounds. If they go beyond them in small numbers 

 they die off. If they go and increase in numbers, the new conditions of life affect their constitution, 

 set in action the principle of change, and they become transformed into new sj^ecies. Again, Mr. 

 Darwin says, in speaking of the Galapagos Archipelago, " The really surprising fact in this 

 case, and in a lesser degree in some analogous instances, is that the new species formed in the 

 separate islands have not quickly spread to the other islands. But the islands, though in sight 

 of each other, are separated by deep arms of the sea, in most cases wider than the British Channel ; 

 and there is no reason to suppose that they have at any former period been continuously united J 

 . . . Many even of the birds, though so well adapted for flj'ing from island to island, are distinct." 



At the Hawaian Islands each separate island has, in a general way, its own set of land shcUs. || 

 At the Sandwich Islands the same thing occur8,§ and even the fishes of difierent islands are said by 

 Agassiz to be distinct from each other.^ Other instances might be given of islands which are 

 favourably situated for receiving immigrants, being inhabited by plants and animals different 

 from those of the neighbouring coasts, although still bearing the far-off impress of a common 

 origin. 



But no stronger instance of the power of a small barrier in retaining species could be cited than 

 that of the straits between the Continent and our own island. The faunas and floras both of 



* Berghaus, " Physikalisohen Atlas." 1845. endemic peculiarity of the species of each individual island 



t Waixace, on " The Physical Geography of the Malay tells of subsequent separation and change wrought in each 



Archipelago," in Proceedings ofRoyal Geographical Society, probably at the same time, by the alteration of climate 



June, 1863. (Separate copy, p. 12.) from continental or terrestrial, to isolated and oceanic. 



I The character and the species of the different islands § Dr. PiciiERiNO, in " Proceedings of American Aca- 



would lead me to a different conclusion. The American demy of Arts and Sciences," vol. iv. p. 193. 1860. 

 type of the whole group speaks primarily of connexion || Dr. A. A. Gould, in "Proceed. Amer. Acad. Ai-ts 



with the continent. The family facies of the group inter and Sciences," vol. iv. p. 195. ISGO. 

 se, speaks of a period when the whole islands were se- T Prof. Agassiz, in "Proceed. Amer. Acad. Arts and 



parated from America, but united to each other. The Sciences," vol. iv. p. 195. 1860. 



D 



