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 few, even of 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 is distinguished from all others in the world by the following constantly associated characters.  They have 1.  A vertebral column; 2.  Mammae; 3.  A placental embryo; 4.  Four legs; 5.  A single well-developed toe in each foot provided with a hoof; 6.  A bushy tail; and 7.  Callosities on the inner sides of both the fore and the hind legs.  The asses, again, form a distinct species, because, with the same characters, as far as the fifth in the above list, all asses have tufted tails, and have callosities only on the inner side of the fore legs.  If animals were discovered having the general characters of the horse, but sometimes with callosities only on the fore legs, and more or less tufted tails; or animals having the general characters of the ass, but with more or less bushy tails, and sometimes with callosities on both pairs of legs, besides being intermediate in other respects—­the two species would have to be merged into one.  They could no longer be regarded as morphologically distinct species, for they would not be distinctly definable one from the other.

However bare and simple this definition of species may appear to be, we confidently appeal to all practical naturalists, whether zoologists, botanists, or palaeontologists, to say if, in the vast majority of cases, they know, or mean to affirm, anything more of the group of animals or plants they so denominate than what has just been stated.  Even the most decided advocates of the received doctrines respecting species admit this.