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Show EMBRYOLOGY. 442 CHAP. XIII. t med to see differences in "Ve are so much accu~ 0 0 and the adult, and like-structure bet';e~n ~he i:~h:yembryos of widely different wise a close similarity 1 s that we might be led . h' the same c as ' animals w1t In ecessarily contingent in some l k t these facts as n . h to oo a h But there is no obv1ous reason w. y, manner on growt ·. f a bat or the fin of a porpOise, · t the wing o ' for Ins ance, b ketched out with all the parts h uld t have een s s o no f as soon as any structure became in prop~r propor I~, And in some whole groups of visible In th~ em rtyaol·n· members of other groups, the · ls and In cer . anima t t any period differ widely from the b yo does no a l fi h em r 0 1 remarked in regard to cutt e- s ' adult : ~hus wetn lasphosis . the cephalopodic character H there IS no me amor ' b . i£ t d long before the parts of the em ryo ~re IS man es ~' aO'ain in spiders, "there is nothing completed ; and b h · " The larvoo of be called a metamorp osis. . ~orthy to her ada ted to the most diverse and active Insects, whet. . Pt' being fed by their parents or habit~ ?r t~mt:i:~co~v~roper nutriment, yet nearly all place h In :a similar worm-like stage of development; pass t roug . that of Aphis if we look to b t . some few cases, as In ' f 1 u m . d . · by Professor Huxley o t 18 the admuable rawings t. ce of the ver-development of this insect, we see no Ia miform stage. ex lain these several facts i~l How, then, can we P . 1 but not um- 1 the very genera ' embryology,-name y b t een the embryo and ~~ · structure e w versal drner nee In . h same indivividual embryo, the adult ;-of parts In t e l'ke and serve for which ultimately b~comet vt~~ ::rlly period of growth diverse purposes, being a . within the same alike ;-of embryos of differe.nt sp~~Ies resembling each class, generally, but not r~:e:.:b~o not being closely other ·-of the structure o t . t except when the related to its conditions of exls ence, CIIAP. XIII. EMBRYOLOGY. 443 embryo becomes at any period of life ~ctive and has to provide for itself ;-of the embryo apparently having sometimes a higher organisation than the mature animal, into which it is developed. I believe that all these facts can be explained, as follows, on the· view of descent with modification. It is commonly assumed, perhaps from monstrosities often affecting the embyro at a very early period, that slight variations necessarily appear at an equally early period. But we have little evidence on this headindeed the evidence rather points the other way; for it is notorious that breeders of cattle, horses, and various fancy animals, cannot positively tell, until some time after the animal has been born, what its merits or form will ultimately turn out. We see this plainly in our own children; we cannot always tell whether the child will be tall or short, or what its precise features will be. The question is not, at what period of life any variation has been caused, but at what period it is fully displayed. The cause may have acted, and I believe generally has acted, even before the embryo is formed; and the variation may be due to the male and female sexual elements having been affected by the conditions to which either parent, or their ancestors, have been exposed. Nevertheless an effect thus caused at a very early period, even before the formation of the embryo, may appear late in life; as when an hereditary disease, which appears in old age alone, has been communicated to the offspring from the reproductive element of one parent. Or again, as when the horns of cross-bred cattle have been affected by the shape of the horns of either parent. For the welfare of a very young animal, as long as it remains in its mother's womb, or in the egg, or as long as it is nourished and protected by its parent, it must be quite unimportant whether most of its characters are fully |
