376 PRINCIPLES AND METHODS OF PALAEON*rOLOGY. 



which they flourished were respectively warm and cold. The force of 

 the conclusion will depend upon the extent of our previous observation 

 with regard to A and B. In practice, and within certain limits, such a 

 conclusion is probably valid, but it is a very different matter if the ar- 

 gument is put, as it more commonly is, thus: Species A and B are 

 ibund respectively in hot and cold climates ; therefore species a, which 

 is very lilvc A, though not the same, and species />, which is very like 

 B, though distinct, indicate that the climates in whicli they flourished 

 were respectively w^arm and cold. 



This argument, it is obvious, is only valid on the assumption that cer- 

 tain amount of simihirity of form implies simihirity of necessary condi- 

 tions; and the question immediately arises: How much similarity of 

 form implies how much similarity of condition? 



In the present state of science no definite answer can be given to this 

 (luestion. It is not understood why some genera are well-nigh universal 

 in their distribution, others limited in their area. No comparison of the 

 osteology of the arctic fox and of the jackal, of the pola» bear and of the 

 black bear, of the musk ox and of the buifalo, would enable the ana- 

 tomist to tell whicli of these species inhabits an arctic, and which a 

 warmer climate. And on the other hand, though the existing species 

 of hippopotamuses, rhinoceroses, and elephants are now exclusively in- 

 habitants of warm climates, it is certain that very similar species 

 formerly flourished in climates at least as cold as that of England, if not 

 much colder. 



That these difliculties beset the enunciation of laws of distribution of 

 general application, indicates what is tolerably certain on other grounds, 

 that the existing arrangement of living beings on the surface of the globe 

 is a complex result, the product of the interaction of a number of distinct 

 causes. It is pretty clear, indeed, from what we know of life, that the 

 presence or absence of any particular living being, on any given spot of 

 the earth's surface, must depend on these conditions: 



1st. The mode and place of origin of that kind of living being. 



2d. Its powers of voluntary migration. 



3d. The extent to which it has undergone involuntary migration in 

 consequence of changes in the distribution of sea and land, currents, t&c. 



4th. The range of climatal and other conditions under which alone it 

 can exist. 



If we had these data for each species, its distribution would be a matter 

 of calculation. But unfortunately they are not yet ascertained for any 

 species whatsoever; nor is there, with regard to one or two, that agree- 

 ment among men of science as to the i^robabilities of the case which 

 would be desirable. 



Thus, respecting the first condition, no one has ever witnessed the 

 origin of a species, nor is there any scientific evidence as to the mode or 

 place of origin of any living thing. 



As to the hypothetical views, all the possil>le alternatives have their 

 advocates. There are those who suppose that all living beings were 

 created at once, in one spot, whence they have subsequently migrated ; 

 but persons of sound intellect, acquainted with the facts, usually attacli 

 themselves to one of two other views. On the one hand, some conceive 

 that all living beings were created as we find them, and where we find 

 them ; or that, at any rate, they are the descendants of a stock created 

 within a distance not greater than can be overcome by the voluntary or 

 involuntary migration of the species. Those who entertain this view 

 usually suppose that a species once created can only be modified to a 

 very limited extent. 



