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§ 2. Evidence of Identity of Species.
Such being the case, the only question is, how can we deter-mine whether the immaterial principle which constitutes and deter-mines the species, be the same or different. Aside from divine revelation, this can be ascertained: (1.) Partly from the organic structure. (2.) Partly from the φύσις, or physical nature. (3.) Partly from the ψυχή, or psychological nature. (4.) Partly from permanence and capability of indefinite propagation.
The first evidence of the identity of species is to be sought in the σῶμα, or the organic structure. The evidence of design is impressed won all the organized bodies in the universe, and especially upon the bodies of all animals. Those intended to live on the dry ground, those intended to live in water, and those intended to fly in the air, have their animal frame adapted to these severe, modes or conditions of existence. There is also clear evidence of 83the unity of this design. That is, it is carried out in all parts of the bodily organization. Those animals intended to live on dry ground have none of the structure, or organs, or members peculiarly suited to aquatic animals. The lion, tiger, ox, horse, etc., have neither the gills, the scales, the fins, nor the rudder-like tail of the fish. All parts of the animal harmonize. They are all related and adapted to one and the same end. The body of the fish is shaped so as to cleave the water with the least resistance; its fins are oars, its tail is adapted both for propulsion and guidance; its breathing apparatus is suited to separate the air from water; its digestive organs are adapted to the assimilation of the kind of food furnished by the element in which it lives. The same thing is obviously true of all terrestrial animals. Besides this general adaptation of animals for living in the air, in the water, and on the dry ground, there are innumerable more specific adaptations suiting the species of fishes, birds, and land animals for the particular modes of life for which they are designed. Some are intended to be carnivorous, and their bodies are harmoniously constructed with a view to that end. Others are intended to live on herbs, and in them we find everything adapted for that purpose. This adaptation refers to numerous and varied purposes. Hence the genera and the species of animals belonging to the different departments, classes, orders, and families into which the animal kingdom is divided, are exceedingly numerous, and each has its distinctive corporeal organization indicative of the specific end it is intended to subserve. So minute, and so fixed is the plan on which each species of animal is constructed, that a skilful naturalist, from the examination of a single bone, can tell not only the family, or genus, but the very species to which it belongs. Agassiz has, from a single scale of a fish, delineated its whole body as accurately as though the living animal had been photographed. And the correctness of his delineation has been afterwards verified by the discovery of a perfect specimen of the species portrayed. Now, the important principle deducible from these admitted facts is, that no diversity of colour, form, proportion, structure, etc., not indicative of design, or not proving a difference in the immaterial principle which determines the nature of the animal, can of itself be admitted as proof of diversity of species. The Italian greyhound and the English mastiff differ in all the respects just mentioned. The Shetland pony, the London dray-horse, and the Arabian or the Barb exhibit similar striking diversities. But when they come to be anatomically examined, it is found that they are constructed on the 84same plan. The bony structures, the distribution of the nerves, muscles, and blood-vessels, are all expressive of the same general intention. Hence, naturalists refer these varieties to the same species. And the correctness of this conclusion is confirmed by every other criterion of the identity of species. While it is admitted that such diversities do exist in the varieties belonging to the same species of the lower animals, it is surprising that far less diversities of the same kind among the varieties of the human family should be insisted upon, as evidence of difference of species. The wild dog wherever found is nearly of the same colour, and the same size, with ears, limbs and tail of the same form, and yet how endless are the permanent varieties derived from that original stock. It is well known that such varieties can be artificially produced. By skilful breeding almost any peculiarity of form, colour, or structure within the limits of the original idea of the species, can be produced and perpetuated; as is seen in the different breeds of horses, cattle, and sheep found even in so restricted a field of operation as Great Britain. It is certain, therefore, that no diversity of an external or material character, not indicative of diversity of design, plan, and intention can properly be assumed as indicative of diversity of species. The presence of a skin connecting the toes or claws of a bird, is in itself a comparatively small affair. It is insignificant as to the amount of material expended, and as to the effect on the general appearance compared to the points of difference between the greyhound and the mastiff, and yet it is indicative of design. It indicates that the animal is intended to live in the water; and everything else in its structure and nature is found to correspond with that intention. A small difference of structure indicative of design will prove difference of species, when much greater differences not thus indicative are perfectly consistent with unity of species.
The second method of determining the identity of the immaterial principle in which the idea of species resides, is the examination of its φύσις, or its physiology. To this department belongs all that relates to enervation or the distribution of the nerve power; to the circulation of the blood; to respiration; to calorification or production of animal heat; to the distribution of the muscles voluntary and involuntary; to the processes of digestion, assimilation, propagation, etc., etc. As to this point it is to be observed, (1.) That the φύσις, or animal nature, is always in accordance with 85the σῶμα, or corporeal structure. We never find the organs of an aquatic animal with the φύσις of a land animal. Everything relating to the physiology of the animal is in harmony with its corporeal organization. (2.) That where in all respects the physical nature of individuals or varieties is the same, there the species is the same; where the φύσις is different, the species is different. (3.) That the physiology of an animal is thus as easily ascertained, and is just as uniform and fixed, as its material structure, and in fact much more so. The material structure may, and as we have seen does, differ exceedingly in the different varieties included under the same species, but the φύσις is always the same. The physiology of the greyhound is identical with that of the mastiff; and that of the Shetland pony is the same as that of the London dray-horse.
The third criterion of the identity of species is to be sought in the ψυχή, or the psychological nature of the animal. The ψυχή is the immaterial principle which belongs to all animals, and is the same in kind in every distinct species. It is that in which the life resides; which is the seat of the instincts, and of that measure of intelligence, be it greater or less, which belongs to the animal. The ψυχή is the same in all the individuals of the same species, and it is permanent. The instincts and habits of the bee, the wasp, the ant, and the beaver; of the lion, tiger, wolf, fox, horse, dog, and ox; and of all the endless diversities of beasts, birds, fishes, and insects, are the same in all ages and in all parts of the world. This immaterial principle is of a higher order in some cases than in others, and admits of greater or less degrees of culture, as seen in the trained elephant or well-disciplined pointer. But the main thing is that each species has its own ψυχή, and that this is a higher element and more decisive evidence of identity than the corporeal structure or even the φύσις, or animal nature. Where these three criteria concur, where the corporeal organization, in everything indicative of design, is the same; where the φύσις and the ψυχή, the physical and psychological natures, are the same, there, beyond all reasonable doubt, the species is the same.
The fourth criterion of species is found not only in its permanence but in the capacity of procreation and indefinite propagation which belongs to all the individuals and varieties which it includes. Animals of the same species can propagate their kind. Animals of different species cannot combine and perpetuate a new or mongrel 86species. This as we have seen is an admitted fact among all classes of naturalists, a few individuals excepted. It is a fact patent to all mankind and verified by the experience of all ages.
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