234 SPECIES mon characters of the individuals which com- pose the species. Again, when the species thus established are compared, certain of them are found to agree with one another, and to differ from all the rest in some one or more peculiarities. They thus form a group, which in one sense is merely a species of higher or- der, hilo technically it is termed a "genus." And, by a continuation of the same process, genera are grouped into families, families into orders, and so on. Each of the groups thus named is in the logical sense a genus, of which the next lower groups constitute the species. The characters on which species are based ne- cessarily depend upon the nature of the bodies classified. Thus, mineral species are founded upon purely morphological characters ; that is to say, they are defined by peculiarities either of form, color, and the like, or of structure; which last term may be used to include both the physical and the chemical characteristics of a mineral. The distinction between a species and a variety is wholly arbitrary, except so far as it is commonly agreed that individuals which differ from others only as terras of a gradual series of modifications belong to the same spe- cies, and are to be considered merely as varie- ties of that species. It is conceivable. that ani- mals and plants should have been known to us only by their remains preserved in museums or in the fossil state. If this had been the case, biological like mineralogical species could have been defined only by morphological char- acters ; that is to say, by the peculiarities of their outward form and inward structure ; and, as a matter of fact, this is the state of our knowledge in respect of a large proportion of the existing fauna and flora of the world, and of all extinct animals and plants. A botanist or a conchologist, who sets to work to arrange a newly received collection, sorts out his plants or his shells according to their likenesses and unlikenesses of form and structure, until he has arranged them into groups of individuals which agree in certain constant characters, and differ only by insignificant features, or by such peculiarities as vary in different individuals in such a manner that an insensible gradation can be traced between those forms which have the peculiarity strongly marked and those in which it is absent. Thus far the considera- tions which guide the biologist in the estab- lishment of species differ in no respect from those which influence the mineralogist. But al- though naturalists have no more direct knowl- edge of any but the morphological characters 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 gener- ation from other living beings is one of the circumstances in their history which most com- pletely differentiate them from minerals; and ideas derived from the study of the phenomena of generation enter 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 morpho- logical 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 generations" the suc- cessive zooids may differ very widely ; but, in- asmuch as the differing forms in these cases proceed from one parentage, 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 do; but, apart from other considera- tions, 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 now be admitted, and that living beings which arise from totally distinct parents must be held to be of different species, even though morphologically identical. The well-nigh exploded hypothesis of the mul- tiplicity of centres of origin for species of wide distribution, indeed, implies the belief that groups of individuals which have proceeded from distinctly created parents may neverthe- less be of the same species ; while the support- ers of the no less nearly extinct hypothesis of the independent creation of the faunas and floras of successive formations used to affirm that, although indistinguishable, two animals or pjants from separate formations must be of distinct species, because they have been created separately. However, these subtleties have ceased to have any practical importance. In the next place, it is observed that, while indi- viduals of the same morphological species breed freely with one another and give rise to per- fectly fertile offspring, the unions of individ- uals of different morphological species are, as a rule, either infertile or imperfectly fertile. Thus fertility, like parentage, has become a physiological character of species; and though in the case of some domesticated anihials, as pigeons, the extreme forms are more different from one another than are many morphologi- cal species, yet, apart from the historical evi- dence of their parentage, they are held to be members of the same species because they are all perfectly fertile one with another, and their offspring are also perfectly fertile. Thirdly, it is a matter of experience that, as a general rule, and taking the whole cycle of forms through which a living being runs into ac- count, offspring and parent are so similar that they belong to one and the same morphological species ; and it is further in evidence that many species have endured for extremely long periods
