It is impossible here to do more than merely indicate in the briefest manner the two fundamental ideas which are at the bottom of all the various theories as to the origin of species; and it will be sufficient to give an outline of the two leading theories, without adducing any of the reasoning upon which they are based. It should be added, however, that almost all scientific men are at the present day agreed that species have been produced by a process of evolution or development, though all are not agreed as to the manner in which that evolution has been carried out.
Doctrine Of Special Creation. On this doctrine of the origin of species it is believed that species are immutable productions, each of which has been specially created at some point within the area in which we now find it, to meet the external conditions there prevailing, subsequently spreading from this spot as far as the conditions of life were suitable for it.
II. Doctrine of Development. - On the other hand, it is believed that species are not permanent and immutable, but that they "undergo modification, and that the existing forms of life are the descendants by true generation of pre-existing forms" (Darwin).
On Lamarck's theory of the development of species, the means of modification were ascribed to the action of external physical agencies, the inter-breeding of already existing forms, and the effects of habit.
The doctrine of the development of species by variation and natural selection - propounded by Darwin, and commonly known as the Darwinian theory - is based upon the following fundamental propositions:
1. The progeny of all species of animals and plants exhibit variations amongst themselves in all parts of their organisation, no two individuals being exactly and in all respects alike. In other words, in every species the individuals, whilst inheriting a general likeness to their progenitors, tend by variation to diverge from the parent-type in some particular or other.
2. Variations arising in any part of the organism, however minute, may be transmitted to future generations, under certain definite and discoverable laws of inheritance.
3. By "artificial selection," or by breeding from individuals possessing any particular variation, man, in successive generations, can produce a breed in which the variation will be permanent, the divergence from the parent-type being usually intensified by the process of inter-breeding. The races thus artificially produced by men are often as widely different as are distinct species of wild animals.
4. The world in which all living beings are placed is one not absolutely unchanging, but is liable, on the contrary, to subject them to very varying conditions.
5. All animals and plants give rise to more numerous young than can by any possibility be preserved, each species tending to increase in numbers in a geometrical progression.
6. As these young are none of them exactly alike in all respects, a process of "natural selection" will ensue, whereby those individuals which possess any variation, however slight, favourable to the peculiarities of the life of the species, will tend to be preserved. Those individuals, on the other hand, which do not possess any such favourable variation, will be placed at a disadvantage in the "struggle for existence," and will tend to be gradually exterminated. The individuals, therefore, composing any species are thus subjected to a rigid process of sifting, by which those least adapted to their environment are being perpetually weeded out, whilst "the survival of the fittest" is secured.
7. Other conditions remaining the same, the individuals which survive in the struggle for existence will transmit the variations to which they owe their preservation, to future generations.
8. By a repetition of this process, "varieties" are first established; these become permanent, and "races" are produced; finally, in the lapse of time, the differences thus caused become sufficiently marked to constitute distinct "species."
9. If we grant that past time has been practically infinite, it is conceivable that all the different animals and plants which we see at present upon the globe, may have been produced by the action of natural selection upon the offspring of a few primordial forms, or, it may be, of a single primitive being.
Originally, Mr Darwin appears to have believed that "natural selection" would alone be found to be a sufficient cause to have given rise to all existing species by a process of evolution from pre-existing forms. In view, however, of certain objections which had been brought forward, Mr Darwin seems to have abandoned this position; and a cause supplementary to "natural selection" was sought for in what Mr Darwin terms "sexual selection." The action of sexual selection in a supposed process of evolution, according to Mr Darwin's views, may be stated in the following two propositions:
a. The males of many species of animals are known to engage in very severe contests for the possession of the females, these latter yielding themselves to the victor. In such contests certain males will inevitably have certain advantages over the others, either in point of strength or activity, or in consequence of the possession of more efficient offensive weapons. There will therefore always be a probability that certain males will get possession of the females in preference to others : and thus there will be a tendency in the individuals of many species of animals to secure a preponderance of offspring from the strongest males. The peculiarities which enable certain males to succeed in these contests will, coeteris paribus, be transmitted to their male offspring, and in this way-variations may be perpetuated, initiated, or intensified.
b. In the preceding cases, the females are believed to be perfectly passive, and the selection is a "natural" one, the final result depending solely upon the natural advantages which certain males possess' over others in actual combat. It is alleged, however, that there are other cases in which the selection is truly "sexual," since its result is determined by spontaneous preference, and not by brute force alone. It is asserted, namely, that among certain species of animals, the females exercise a free choice as to the particular male with which they will pair; the males being passive agents in the matter, except in so far as each uses, Or may use, his utmost exertions to secure that the choice of the female may fall upon him. The circumstances supposed to influence, and ultimately determine, the choice of the female, are of course, in the main, the personal attractions of some particular male, the female being captivated by some "beauty of form, colour, odour, or voice," which such a male may possess.
If it be admitted that the females of some of the lower animals have the power of expressing and exercising a preference in the manner above indicated, then it is easy to understand how variations might be transmitted or intensified in this way. The male who is most attractive to the female will, other things being equal, have the best chance of propagating his species, and is likely to leave the largest number of descendants. His male offspring will inherit the peculiarities by which their sire was rendered pre-eminently attractive in the eyes of their mother, and thus a well-marked breed might be produced, by the preservation or intensification of characters of this nature. Mr Darwin is disposed to believe that colour and song in most, if not in all animals, are thus to be ascribed to the action of sexual selection, through numerous successive generations; but other competent authorities are unable to concur in this view.