"A great number of facts show, as will be more fully insisted on, that when its will prompts an animal to this or that action, the organs which are to execute it receive an excess of nervous fluid, and this is the determinant cause of the movements necessary for the required action. Modifications acquired in this way eventually become permanent in the breed that has acquired them, and are transmitted to offspring, without the offspring's having itself gone through the processes of acquisition which were necessary in the case of the ancestor.[312] Frequent crosses, however, with unmodified individuals, destroy the effect produced. It is only owing to the isolation of the races of man through geographical and other causes, that man himself presents so many varieties, each with a distinctive character. "A review of all existing classes, orders, genera, and species would show that their structure, organs, and faculties, are in all cases solely attributable to the surroundings to which each creature has been subjected by nature, and to the habits which individuals have been compelled to contract; and that they are not at all the result of a form originally bestowed, which has imposed certain habits upon the creature.[313] "It is unnecessary to multiply instances; the fact is simply this, that all animals have certain habits, and that their organization is always in perfect harmony with these habits.[314] The conclusion hitherto accepted is that the Author of Nature, when he created animals, foresaw all the possible circumstances in which they would be placed, and gave an unchanging organism to each creature, in accordance with its future destiny. The conclusion, on the other hand, here maintained is that nature has evolved all existing forms of life successively, beginning with the simplest organisms and gradually proceeding to those which are more complete. Forms of life have spread themselves throughout all the habitable parts of the earth, and each species has received its habits and corresponding modification of organs, from the influence of the surroundings in which it found itself placed.[315] "The first conclusion supposes an unvarying organism and unvarying conditions. The second, which is my theory (_la mienne propre_), supposes that each animal is capable of modifications which in the course of generations amount to a wide divergence of type. "If a single animal can be shown to have varied considerably under domestication, the first conclusion is proved to be inadmissible, and the second to be in conformity with the laws of nature." This is a milder version of Buffon's conclusion (see _ante_, pp. 90, 91). It is a little grating to read the words "la mienne propre," and to recall no mention of Buffon in the 'Philosophie Zoologique.'