gradation can be traced between those forms which have the peculiarity strongly marked and those in which it is absent.
Thus far the considerations which guide the biologist in the establishment of species differ in no respect from those which influence the mineralogist.
But although naturalists have no more direct knowledge of any but the morphological character of the great majority of the species of animals and plants than they would have of so many mineral specimens, they are familiar with many animals and plants in the living state when they exhibit phenomena to which the mineral world presents no parallel, and the study of these phenomena of active life has complicated the conception of species in biology, by adding physiological to morphological considerations.
The fact that living beings originate by generation from other living beings is one of the circumstances in their history which most completely differentiates them from minerals. This process of generation enters in various ways into the conception of biological species.
For example, it is a generally assumed axiom in biology that whatever proceeds from a living being by way of generation is of the same species as that from which it proceeds, whether the morphological differences between parent and offspring be great or small. The two sexes are often extraordinarily different, and in cases of the so-called alternation of generation the successive zoöids may differ very widely; but, inasmuch as the differing forms in these cases proceed from the same parents, no one doubts that they belong to the same species. The breeds of domesticated animals and plants often differ morphologically as widely as admitted species, but, apart from other considerations, historical evidence that they have the same parentage suffices to cause them to be regarded as of one species. It is not quite clear that the converse of the axiom which has just been referred to would be admitted, and that living beings which arise from totally distinct parents are of different species, even though morphologically identical. The wellnigh exploded hypothesis of the multiplicity of centres of origin for species of wide distribution implies the belief that groups of individuals which have proceeded from distinctly-created parents may, nevertheless, be of the same species, while the supporters of the no less nearly extinct hypothesis of the independent creation of the fauna and flora of successive formations used to affirm that, although indistinguishable, two forms from separate formations must be of distinct species, because they had been created separately. However, these subtilties have ceased to have any practical importance.
In the next place it is observed that, while individuals of the same morphological species breed freely with one another and give rise to perfectly fertile offspring, the unions of individuals of different morphological species are, as a rule, either unfertile or imperfectly fertile.
