Take, for instance, the similarity above alluded to between the fauna of the Arctics and that of the Alps, certainly showing a direct relation between climatic conditions and animal and vegetable life.  Yet even there, where the shades of specific difference between many animals and plants of the same class are so slight as to battle the keenest investigators, we have representative types both in the Animal and Vegetable Kingdoms as distinct and peculiar as those of widely removed and strongly contrasted climatic conditions.  Shall we attribute the similarities and the differences alike to physical causes?  Compare, for example, the Reindeer of the Arctics with the Ibex and the Chamois, representing the same group in the Alps.  Even on mountain-heights of similar altitudes, where not only climate, but other physical conditions would suggest a recurrence of identical animals, we do not find the same, but representative types.  The Ibex of the Alps differs, for instance, from that of the Pyrenees, that of the Pyrenees from those of the Caucasus and Himalayas, these again from each other and from that of the Altai.

But perhaps the most conclusive proof that we must seek for the origin of organic life outside of physical causes consists in the permanence of the fundamental types, while the species representing these types have differed in every geological period.  Now what we call typical features of structure are in themselves no more stable or permanent than specific features.  If physical causes, such as light, heat, moisture, food, habits of life, etc., acting upon individuals, have gradually in successive generations changed the character of the species to which they belong, why not that of the class and the branch also?  If we judge this question from the material side at all, we must, in order to judge it fairly, look at it wholly from that point of view.  If these specific changes are brought about in this way, it is because external causes have positive permanent effects upon the substances of which animals are built:  they have power to change their hair, to change their skin, to change certain external appendages or ornamentations, and any other of those ultimate features which naturalists call specific characters.  Now I would ask what there is in the substances out of which class characters are built that would make them less susceptible to such external influences than these specific characters.  In many instances the former are more delicate, more sensitive, far more fragile and transient in their material nature than the latter.  And yet never, in all the chances and changes of time, have we seen any alteration in the mode of respiration, of reproduction, of circulation, or in any of the systems of organs which characterize the more comprehensive groups of the Animal Kingdom, although they are quite as much under the immediate influence of physical causes as those structural features which have been constantly changing.