HISTORY OF THE DARWINIAN THEORIES.
§ 1. Darwin.
In order to explain the development of higher species from lower ones in a natural way, Darwin starts from two facts. The first fact is, that all individuals of the same species show, besides their specific similarity, individual differences: a fact which we call the law of individual variability. The other fact is, that each individual inclines to transmit to his offspring all his qualities—not only the characteristics of the species, but also those of the individual: a fact which we call the law of heredity.
To show how the whole basis of explanation of the evolution of one species from another is given in these two facts, Darwin calls attention to the rules according to which the often extraordinarily great varieties of domestic animals and cultivated plants are obtained and preserved; namely, the rules of artificial breeding. The breeder simply selects from a species those individuals having such individual qualities as he wishes to preserve and to increase, and refrains from breeding those individuals which do not possess the characteristics he wants or which possess them only in a small degree. He continues the same process with the next generation; and by the constant and effectual agency of the two
before-mentioned laws, he will, after the lapse of a few generations, have breeded a variety in which the characteristics originally belonging only to a single individual have become common and permanent.
It is now important to consider whether nature, in natural selection (whence the name "Selection Theory") does not act unconsciously according to the same rules, and attain the same results, as man with his artificial and intentional selection; and, furthermore, whether she does not reach results which, according to that principle of natural selection, finally explain the origin of all, even of the highest and most complicated organisms, from one single original form or a few original and simplest forms. Darwin finds these questions answered in the affirmative; and arrives at this answer through the following conclusions.
The English political economist Malthus (1766-1834), in his "Essay on the Principles of Population," established a law in regard to the growth of the human race, which may be applied just as well to all the species of the entire organic world: that population tends to increase in a geometrical ratio, although the conditions of life for the individual remain the same or at most increase in an arithmetical ratio. The consequence is that if the species is to be preserved and the individuals of future generations are to continue to find sufficient food and other means for sustaining life, a great many individuals of each generation must perish very early, and even as germ and seed, and only a minority will be preserved and reproduced. This exuberant prodigality of life-germs, of which proportionately only a few are preserved and reproduced, takes place in the plant and
animal world in a very marked degree. There a continual struggle for existence prevails; each individual has to get access to his conditions of life by wresting them from a whole series of other individuals of his own or other species; and now the question arises: which individuals will survive in this struggle? which will more probably be preserved and procreate offspring? Evidently, the answer is, those individuals which possess individual characteristics more favorable to the preservation of the individual than those possessed by other individuals. These individual characteristics are transmitted to the next generation. In this there will be again individuals that have in a still higher degree the characteristics thus transmitted and favorable for the preservation of the individual, or that add to these favorable characteristics new characteristics favorable in another direction to the survival of the individual in the struggle for existence. While these individuals, with more probability than the others, are thus preserved and reproduced, they transmit to their offspring not only the old favorable characteristics increased, but also those newly added. Among the favorable individual qualities, Darwin reckons the divergence of character, the perfection of organization, and the law of correlation; the latter, however, can not be explained by natural selection, since according to this law a variation in an organ brings about a corresponding variation in entirely different organs (e.g., cats with white fur and blue eyes are also deaf).
This is natural selection by means of the survival of the fittest in the struggle for existence. Changes in the conditions and surroundings of life, and more or less
perfect adaptation of the organisms to the new conditions of form, color, food and habit, are the main causes of those individual variations, the accumulation of which through many generations produces so great effects. If we only have behind us periods long enough to permit us to imagine each step in the development as an extremely small and hardly appreciable one, natural selection offers us not the exclusive but the main means of explaining the evolution of the whole animal and plant world out of one or a few simple organized original forms.