394 = PEO VISIONAL HYPOTHESIS Chap. XXVII. 



For instance, two embryos might thus come to have on their 

 adjoinmg 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 already 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 gem- 

 mules 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 foi 



single part ; or it may be that the gemmules from one of 

 adjoining embryos alone become developed 



Variability often depends, as I have attempted to show 

 on the reproductive organs being injuriously affected 

 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 

 as a plant, if its own pollen be excluded, is easily hybridised, so, 

 in 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 developed 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 

 inherited gout, &c, the tissues of the body, according to the 

 doctrine of pangenesis, are directly affected by the new con- 



• 



ditions, and consequently throw off modified gemmules, which 



51 Muller's ' Physiology,' Eng. translat., vol. i. p. 407. 





