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Show 92 THE PEltrE'I'UATIO~ OF LIVING BEINGS, been very carefully studied. Now, the curious thing is th1s, that although you have the same elements in the experiment in each case, the offspring is entirely different in character, according as the male influence comes from the Ass or the Horse. 'iVhere the Ass is the male, as in the case of the :M:ule, you find that the head is like that of the Ass, that the ears are long, the tail is tufted at the end, the feet are small, and the Yoice is an unmistakable bray; these are all points of sin1ilarity to the Ass; but, on the other band, the barrel of the body and the cut of the neck are much more like those of the Mare. Then, if you look at the Hinny,-thc result of the union of the Stallion and the she-Ass, then you find it is the I-Iorse that has the predominance; that the head is more like that of the 1-Iorse, the ears are shorter, the legs coarser, and the ty~e is altogether altered; while the voice, instead of be1ng a bray, is the ordinary neigh of the Horse. Here, you see, is a 1nost curious thing: you take exact:y the same elements, Ass and Horse, but you ~ombine the sexes in a different manner, and the result 1S modified accordingly. You have i l this case, how~ ver, a result which is not general and universal-there lS usually an important preponderance, but not always on the same side. Here, then, is one intelligible, and, perhaps, necessary cause of variation: the fact, that there are t\;vo sexes sharing in the production of th e oCuf!s pn• ng, an d that the share taken by each is different and variable not only for each combination, but also for di:fferen~ members of the same family. HEREDI'I'ARY TltANSMISSION AND VARIATIO)I". 93 Secondly, there is a variation, to a certain extent,though in all probability the influence of this cause has been very much exaggerated-but there is no doubt that variation is produced, to a certain extent, by what are commonly known as external conditions,-such as temperature, food, warmth, and moisture. In the long run, every variation depends, in some sense, upon external conditions, seeing that everything has a cause of its own. I use the term "external conditions" now in the sense in which it is ordinarily employed : certain it is, that external conditions have a definite effect. You may take a plant which has single flowers, and by dealing with the soil, and nourishment, and so on, you may by-and-by convert single flowers into double flowers, and make thorns shoot out into branches. You may thicken or make various modifications in the shape of the fruit. In animals, too, you may produce analogous changes in this way, as in the case of that deep bronze eolour which persons rarely lose after having passed any length of time in tropical countries. You may also alter the development of the muscles very much, by dint of training; all the world knon· ' that exercise has a great effect in this way; we always expect to find the arn1 of a blacksmith hard and wiry, and possessing a large development of the brachial 1nuscles. No doubt, training, which is one of the forms of external conditions, converts what are originally only instructions, teachings, into habits, or, in other words, into organizations, to a great extent; but thi second cause of variation cannot be considered to be by any means a large one. The third cause that I |