OCR Text |
Show 176 SUMMARY OF TilE CHAP. XIX. ·a 1. ·a . babeat qnod hujus-crossing, asks, "an I a Iqm m reces~u • . · lvere sed semper eo cemod1 flores nunquam propno suo pu ' . . . . . t . merito qurentur? Certe aharum sure speciel Impregnen ur, .. natura m.l 1~ ae1. t f rus t ra. , Altl1 oug h we may d. emur . to Kol- renter's saym. g that nature d oes no tl n·n g in vam • seemg how many orgam.c bem. gs retam· ru d1 · m en tary . and useless organ. s' d bt dl th ment from the mnumerable contnv-vet un ou e y e argu . . . ~nces, which favour the crossing of distinct mdtvidu~ls of the same speci·e s, I·S of t 11 e grea t est wei·g · ht· Th. e most Importan. t result of this law is that it leads to uniformity of character ~n the individuals of the same species. In the case 0~ ce1·tam hermaphrodites, which probably intercross ?~ly at long mtervals of time, and with unisexual animals inbab1tmg som.ewhat separated localities, which can only occasionally come mto cont~ct and pair, the greater vigour and fert.ility ?f the crossed offsprmg will ultimately prevail in giving umformity of character to the individuals of the same species. But when \Ye go beyond the limits of the same species, free intercrossing is barred by the law of sterility. . In searchino- for facts which might throw hght on the cause of the good effec~s from crossing, and of the evil effe~ts .from ~lose interbreeding, we have seen that, on the one band, It IS a widely prevalent and ancient belief t?~t anim~ls and pl~nts profit from slio·ht changes in their conditiOn of hfe ; and It would appear th:t the germ, in a somewhat analogous manner, is more effect~ally stimulated by the male element, when taken from a distinct individual, and therefore slightly modified in nature, than when taken from a male having the same identical constitution. On the other hand, numerous facts have been given, showing that when animals are first subjected to captivity, even in their native land, and although allowed much liberty, their reproductive functions are often greatly impaired or quite annulled. Some groups of animals are more affected than others, but with apparently capricious exceptions in every group. Some animals never or rarely couple: some couple freely, but never or rarely conceive. The secondary male characters, the maternal functions and instincts, are occasionally affected. With plants, when first subjected to cultivation, analogous facts have been observed. We probably owe our double flowers, rich seedless CHAP. XIX. FOUR LAST CHAPTERS. 177 fruits, and in some cases greatly developed tubers, &c., to incipient sterility of the above nature combined with a copious supply of nutriment. Animals which have long been domesticated, and plants which have long been cultivated, can generally withstand with unimpaired fertility great changes in their conditions of life; though both are sometimes sbghtly affected. With animals the somewhat rare capacity of breeding freely under confinement has mainly determinerl, together with their utility, the kinds which have been domesticated. Vve can in no case precisely say what is the cause of the diminished fertility of an animal when first captured, or of a plant vvhen first cultivated; we can only infer that it is caused by a change of some kind in the natural conditions of life. The remarkable susceptibility of the reproductive system to such changes,-a susceptibility not common to any other organ,apparently has an important bearing on Variability, as we shall see in a future chapter. It is impossible not to be struck with the double parallelism between the two classes of facts just alluded to. On the one hand, slight changes in the conditions of life, and crosses between slightly modified forms or varieties, are beneficial as far as prolificness and constitutional vigour are concerned. On the other band, changes in the conditions greater in degree, or of a different nature, and crosses between forms whieh have been slowly and greatly modified by natural means,-in other words, between species,-are highly injurious, as far as the reproductive system is concerned, and in some few instances as far as constitutional vigour is concerned. Can this parallelism be accidental? Does it not rather indicate some real bond of connection? As a fire goes out unless it be stirred up, so the vital forces are always tending, according to Mr. Herbert Spencer, to a state of equilibrium, unless disturbed and renovated through tbe action of other forces. In some few cases varieties tend to keep distinct, by breeding at different periods, by great differences in size, or by se-xual preference,-in this latter respect more especially resembling species in a state of nature. But the actual crossing of varieties, far from diminishing, generally adds to the fertility of both the first union and the mongrel offspring. Whether all VOL. II. N |