INTEODL'CTION. XXTll 



nected with that department of Zoology which 

 treats of the Distribution of animal beings. For 

 if it can be shown that animals are capable of be- 

 coming modified to an indefinite extent by the 

 physical conditions under which they are placed, 

 the following conclusions are inevitable : 



1. That many of the apparently dissimilar ani- 

 mal forms found in difi'erent regions of the globe, 

 are to be viewed as varieties of the same species, 

 the differences between them being due to cor- 

 responding variations in the external agencies to 

 which each has respectively been subjected. 



2. That one species may pass into another, or, 

 in other words, that species have no existence. 



But since little positive evidence can be lu^ged 

 in favour of these conclusions, it seems desirable, 

 for the present at least, to reject them as unsatis- 

 factory, " for it is more probable that species 

 should have been created with a certain decrree of 

 variability, than that mutability should be a part 

 of the scheme of nature." It cannot, however, be 

 denied ^ hat the tendency of several species to 

 form varieties, is much greater than many natu- 

 ralists are accustomed to admit, ^o 



In considering the subject of distribution, it is 

 necessary, as far as possible, to separate the facts 

 of the science from the various theories which 

 have been devised to explain them. The former, 

 if rightly observed, may be accepted, since they 



