258 LAY SERMONS, ADDRESSES, AND REVIEWS. [xn. 



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 w^hich 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 n 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 ruay 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 con- 

 veniently speak of these two senses, or aspects, of 

 " species " — the one as morpholgical, the other as phy- 

 siological. Eegarded 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 mearly sexual, morphological peculiar- 

 ities. Thus horses form a species, because the group of 

 animals to which that name is ap>plied is distinguished 

 frorn all others in the world by the following constantly 



