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which ultimately become very unlike and serve for diverse purposes, being at an early period of growth alike;— the general, but not invariable, resemblance between the embryos or larvæ of the most distinct species in the same class;— the embryo retaining whilst within the egg or womb, structures which are of no service to it, either at that period or later in life; whilst embryos at a later period, or larvæ, which have to provide for their own wants, are perfectly adapted to the surrounding conditions;— and lastly the fact of certain larvæ standing higher in the scale of organisation than the mature animals into which they are developed. I believe that all these facts can be explained, as follows. .. .. .. .. .. .. ..
It is commonly assumed, perhaps from monstrosities .. affecting the embryo at a very early period, that slight variations or individual differences necessarily appear at an equally early period. We have little evidence on this head, but what we have certainly points the other way; for it is notorious that breeders of cattle, horses, and various fancy animals, cannot positively tell, until some time after birth, what the merits or form of their young animals will turn out. We see this plainly in our own children; we cannot .. tell whether a child will be tall or short, or what its precise features will be. The question is not, at what period of life each variation may have been caused, but at what period the effects are displayed. The cause may have acted, and I believe generally has acted, on one or both parents before reproduction.
Text in this page (from paragraph 4300, sentence 600 to paragraph 4300, sentence 700, word 1) is not present in 1869
which ultimately become very unlike and serve for diverse purposes, being at this early period of growth alike;— of embryos of different species within the same class, generally, but not universally, resembling each other;— of the structure of the embryo not being closely related to its conditions of existence, except when the embryo becomes at any period of life active 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 embryo at a very early period, that slight variations necessarily appear at an equally early period. But we have little evidence on this head— indeed 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