The cell : outlines of general anatomy and physiology / by Oscar Hertwig ; translated by M. Campbell ; and edited by Henry Johnstone Campbell.

  • Campbell M.
Date:
1895
Catalogue details

Licence: Public Domain Mark

Credit: The cell : outlines of general anatomy and physiology / by Oscar Hertwig ; translated by M. Campbell ; and edited by Henry Johnstone Campbell. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    differences by producing a new individual, which occupies a mean position between its parents. By this means numberless new varieties are developed, which only differ slightly from one another. Hence Weismann (IX. 34) is of opinion that fertilisa- tion is an arrangement by means of which an enormous number of varying individual combinations arise; these supply the materia] for the operation of natural selection, the result being that new varieties are produced. Whilst agreeing with the first part of this principle, I cannot support the second. The individual differences which are called into being by fertilisation, and which furnish the basis for natural selection, are as a rule only of an insignificant nature, and are always liable to become suppressed, weakened, or forced into another direction, by some subsequent union. A new variety can only be formed, if numerous members of a species vary in a definite direction, so that a summation or strengthening of their peculiarities is arrived at, whilst other individuals of the same species, which preserve their original characters, or vary in another direction, must be prevented from uniting sexually with them. Such a process presupposes the presence of an environment which always acts in a constant manner, and the existence of a certain intervening space between the two sets of individuals belonging to the species, which is destined to divide into two new species. Sexual reproduction, therefore, seems to me to influence the formation of a species in a manner opposed to that suggested by Weismann. By creating intermediate forms, it continually re- conciles the differences which are produced by external circum- stances in the individuals of a species; thus it tends to make the species homogeneous and to enable it to retain its own peculiar features. Here, too, sexual affinity, that mysterions property of organic substance, by preventing a combination, or at any rate a successful one, between substances which are either too similar or too dissimilar, acts as an important factor. For, if the sexual products, on account of their different organisation and their slight sexual affinity, cannot mingle successfully, the species and orders in question are kept apart. Darwin and Spencer express the same opinion. According to the former, “intercrossing plays a very important part in nature, in keeping the individuals of the same species or of the variety true and uniform in character.” And Spencer remarks : “ In a species there is, through gamogenesis, a perpetual neutralization of those