[161]

Dec 21 Hedysarum montanum

[data not transcribed]

Dec 22

[data not transcribed]

(The other seedlings I am sure had cots, more open & now almost closed)

Dec 23

[data not transcribed]

[161v]

15

Chap VII

are governed by highly complex laws, as in the unequal fertility of the reciprocal crosses of between the same two species. We know that with forms which must all be ranked as undoubted species, a perfect series can be formed from those which are absolutely sterile to those which are quite or almost quite fertile when crossed. Nor does any close relation exist between the amount of their difference in external structure or in habits & the degree of their sterility. of the forms which are crossed. It might further be argued that man in many respects may properly be compared with those animals which have long been domesticated, & a large body of evidence can be advanced in favour of the Pallasian doctrine *13 (14) that domestication tends to eliminate that sterility which is so general a result of crossing with species in a state of nature. when intercrossed. Therefore it

[Descent 1: 222: "We know that these qualities are easily affected by changed conditions of life or by close inter-breeding, and that they are governed by highly complex laws, for instance that of the unequal fertility of reciprocal crosses between the same two species. With forms which must be ranked as undoubted species, a perfect series exists from those which are absolutely sterile when crossed, to those which are almost or quite fertile. The degrees of sterility do not coincide strictly with the degrees of difference in external structure or habits of life. Man in many respects may be compared with those animals which have long been domesticated, and a large body of evidence can be advanced in favour of the Pallasian doctrine13 that domestication tends to eliminate the
13 'The Variation of Animals and Plants under Domestication,' vol. ii. p. 109. I may here remind the reader that the sterility of species when crossed is not a specially-acquired quality; but, like the incapacity of certain trees to be grafted together, is incidental on other acquired differences. The nature of these differences is unknown, but they relate more especially to the reproductive system, and much less to external structure or to ordinary differences in constitution. One important element in the sterility of crossed species apparently lies in one or both having been long habituated to fixed conditions; for we know that changed conditions have a special influence on the reproductive system, and we have good reason to believe (as before remarked) that the fluctuating conditions of domestication tend to eliminate that sterility which is so general with species in a natural state when crossed. It has elsewhere been shewn by me (ibid. vol. ii. p. 185, and 'Origin of Species,' 5th edit. p. 317) that the sterility of crossed species has not been acquired through natural selection: we can see that when two forms have already been rendered very sterile, it is scarcely sterility which is so general a result of the crossing of species in a state of nature. From these several considerations, it may be justly urged that the perfect fertility of the intercrossed races of man, if established, would not absolutely preclude us from ranking them as distinct species."]