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Show 394 PROVISIONAL HYPOTHESIS CHAP. XXVIL adjoining sides a single symmetrical arm, which in one sense will have been formed by the fusion of the bones, muscles, &c., belonging to the arms of both embryos. In the case of the fish described by Lereboullet, in which a double head was seen gradually to fuse into a single one, the same process must have taken place, together with the absorption of all the parts which had been alrearly formed. These cases are exactly the reverse of those in which a part is doubled either spontaneously or after an injury; for in the case of doubling, the superabundant gemmules of the same part are separately developed in union with adjoining points; whilst in the case of fusion the gemmules derived from two homologous parts become mingled .• and form a single part; or it may be that the gemmules from one of two adjoining embryos alone become developed. Variability often depends, as I have attempted to show, on the reproductive organs being injuriously affected by changed conditions; and in this case the gemmules derived from the various parts of the body are probably aggregated in an irregular manner, some superfluous and others deficient. Whether a superabundance of gemmules, together with fusion during development, would lead to the increased size of any part cannot be told; but we can see that their partial deficiency, without necessarily leading to the entire abortion of the part, might cause considerable modifications; for in the same manner ~s a plant, if its own pollen be excluded, is easily hybridised, so, m the case of a cell, if the properly succeeding gemmules were absent, it would probably combine easily with other and allied gemmules. We see this in the case of imperfect nails growing on the stumps of amputated fingers,51 for the gemmules of the nails have manifestly been deve]oped at the nearest point. In variations caused by the direct action of changed conditions, whether of a definite or indefinite nature, as with the fleeces of sheep in hot countries, with maize grown in cold countries with inher~ted gout, &c., the tissues of the body, according ;o the d?~trme of pangenesis, are directly affected by the new conditiOns, and_ consequently throw off modified gemmules, which are transmitted with their newly acquired peculiarities to the offspring. On any ordinary view it is unintelligible how changed .; J MiillcT's ' l'hysiology,' Eng. tmnslnt .. , vol. i. p. 407. CHAP. XXVII. OF PANGENESIS. 395 co~ditions, whether acting on the embryo, the young or adult ammal, can cause inherited modifications. It is equally or even more unintelligible on any ordinary view, how the effects of the long-continued use or disuse of any part, or of changed habits of body or mind, can be inherited. A more perplexing problem can hardly be proposed; but on our view we have only to suppose that certain cells become at last not only functionally but stru~turally modified ; and that these throw off similarly modified gemmules. This may occur at any period of developme~ t, and the modific~tion will be inherited at a corresponding perwd ; . for the modified gemmules will unite in all ordinary cases With the proper preceding cells, and they will consequently be developed at the same period at which the modification first arose. With respect to mental habits or instincts, we are so profoundly ignorant on the relation betw.een the brain and the power of thought that we do not know whether an inveterate habit or trick induces any change in the nervous system ; but when any habit or other mental attribute, or insanity, is inherited, we must believe that some actual modification is transmitted; 52 and this implies, according to our hypothesis, that gemmules derived from modified nerve-cells are transmitted to the offspring. It is generally, perhaps always, necessary that an organism should be exposed during several generations to changed conditions or habits, in order that any modification in the structure of the offspring should ensue. This may be partly due to the changes not being at first marked enough to catch the attention, but this explanation is insufficient; and I can account for the fact, only by the assumption, which we shall see under the_ head of reversion is strongly supported, that gemmules denved from each cell before it had undergone the least modific~ tion are transmitted in large numbers to successive generatwns, but that the gemmules derived from the same cells after modification, naturally go on increasing under the same favouring conditions, until at last they become sufficiently numerous to overpower and supplant the old gemmules. Another difficulty may be here noticed; we have seen that 52 See some remarks to this effect by Sir H. Holland in his ' Medical Notes ' 1839, p. 32. ' |