Chap. XXVII. OF PANGENESIS. 383 



between a host of gemmules and nascent cells, we need not 

 feel at all surprised that the commixture of gemmules derived 

 from two distinct species should lead to partial or complete 

 failure of development. With respect to the sterility of 

 hybrids produced from the union of two distinct species, it 

 was shown in the nineteenth chapter that this depends ex- 

 clusively on the reproductive organs being specially affected ; 

 but why these organs should be thus affected we do not 

 know, any more than why unnatural conditions of life, 

 though compatible with health, should cause sterility ; or 

 why continued close interbreeding, or the illegitimate unions 

 of heterostyled plants, induce the same result. The con- 

 clusion that the reproductive organs alone are affected, and 

 not the whole organisation, agrees perfectly with the un- 

 impaired or even increased capacity in hybrid plants for 

 propagation by buds ; for this implies, according to our 

 hypothesis, that the cells of the hybrids throw off hybridised 

 gemmules, which become aggregated into buds, but fail to 

 become aggregated within the reproductive organs, so as 

 to form the sexual elements. In a similar manner many 

 plants, when placed under unnatural conditions, fail to 

 produce seed, but can readily be propagated by buds. We 

 shall presently see that pangenesis agrees well with the 

 strong tendency to reversion exhibited by all crossed animals 

 and plants. 



Each organism reaches maturity through a longer or 

 shorter course of growth and development : the former term 

 being confined to mere increase of size, and development 

 to changed structure. The changes may be small and 

 insensibly slow, as when a child grows into a man, or many, 

 abrupt, and slight, as in the metamorphoses of certain 

 ephemerous insects, or, again, few and strongly-marked, as 

 with most other insects. Each newly formed part may be 

 moulded within a previously existing and corresponding 

 part, and in this case it will appear, falsely as I believe, to 

 be developed from the old part ; or it may be formed within 

 a distinct part of the body, as in the extreme cases of 

 inetagenesis. An eye, for instance, may be developed at a 

 spot where no eye previously existed We have also seen 



