THE ORIGIN OF SPECIES 145 



will, without doubt, discover this bond, but it is often hard 

 to find. 



Again, from sheer want of room, much has to be taken for 

 granted which might readily enough be proved ; and 

 hence, while the adept, who can supply the missing links 

 in the evidence from his own knowledge, discovers fresh 

 proof of the singular thoroughness with which all difficulties 

 have been considered and all unjustifiable suppositions 

 avoided, at every reperusal of Mr. Darwin's pregnant 

 paragraphs, the novice in biology is apt to complain of the 

 frequency of what he fancies is gratuitous assumption. 



Thus while it may be doubted if, for some years, any one 

 is likely to be competent to pronounce judgment on all the 

 issues raised by Mr. Darwin, there is assuredly abundant 

 room for him, who, assuming the humbler, though perhaps 

 as useful, office of an interpreter between the Origin of 

 Species and the public, contents himself with endeavouring 

 to point out the nature of the problems which it discusses ; 

 to distinguish between the ascertained facts and the 

 theoretical views which it contains ; and finally, to show 

 the extent to which the explanation it offers satisfies the 

 requirements of scientific logic. At any rate, it is this 

 office which we purpose to undertake in the following 

 pages. 



It may be safely assumed that our readers have a general 

 conception of the nature of the objects to which the word 

 " species " is applied ; but it has, perhaps, occurred to a 

 few, even to those who are naturalists ex professo, to reflect, 

 that, as commonly employed, the term has a double sense 

 and denotes two very different orders of relations. When 

 we call a group of animals, or of plants, a species, we may 

 imply thereby, either that all these animals or plants have 

 some common peculiarity of form or structure ; or, we may 

 mean that they possess some common functional character. 

 That part of biological science which deals with form and 

 structure is called Morphology that which concerns itself 

 with function, Physiology so that we may conveniently 

 speak of these two senses, or aspects, of " species " the 

 one as morphological, the other as physiological. Regarded 

 from the former point of view, a species is nothing more 

 than a kind of animal or plant, which is distinctly definable 

 from all others, by certain constant, and not merely sexual, 

 morphological peculiarities. Thus horses form a species, 

 because the group of animals to which that name is applied 



