382 



PROVISIONAL HYPOTHESIS 



Chap. XXVII 



ted by a direct transform 



such 



they may first be converted 



cells, and 

 o osseous 



a 



and then into medullary 



the 



erted 

 perm 



marrow and 



lastly, they may first be 

 o bone. So variable am 



itations of these tissues, in themselves so nearly 

 allied, and yet in their external appearance so completely 

 distinct." But as these tissues thus change their nature at 



any age, without any obvious change in their nutrition, we 

 must suppose in accordance with our hypothesis that gemmules 

 derived from one kind of tissue combine with the cells of another 

 kind, and cause the successive modifications. 



It is useless to speculate at what period of development each 



ts off its gemmules ; for the whole subject of the 



elemental tissues is as yet involved 



development of tl 

 in much doubt. Some physiologists, for instance, maintain that 

 muscle or nerve-fibres are developed from cells, which are 

 afterwards nourished by their own proper powers of absorption ; 

 whilst other physiologists deny their cellular origin ; and Beale 

 maintains that such fibres are renovated exclusively by the 



of fresh germinal matter (that is the so-called nuclei) 



formed material 



However this may be, it appears pro- 

 bable that all external agencies, such as changed nutrition, 

 increased use or disuse, &c, which induced any permanent modi- 

 fication in a structure, would at the same time or previously act 

 on the cells, nuclei, germinal or formative matter, from which 



the 



question 



developed, and 



quently 



would act on the gemmules or cast-off atoms 



There is another point on which it is useless to speculate, 

 namely, whether all gemmules are free and separate, or whethei 

 some are from the first united into small aggregates. A feather, 

 for instance, is a complex structure, 



is liable to inherited 

 certainly generates a 



structure, and, as each separate part 

 variations, I conclude that each feather 

 large number of gemmules: but it is 



possible that these may be aggregated into a compound gem 



mule. The same remark 



ppl 



to the petals of a flower 



which in some cases are highly complex, with each ridge and 

 hollow contrived for special purposes, so that each part must 

 have been separately modified, and the modifications transmitted ; 



consequently, separate gemmules, according 



hypothesi 







