Now few of the physiologists who use this language really suppose that the parent of insect with the metamorphosed jaw, was an insect with [more] so many legs, or that the parent of flowering plants, originally had no stamens, or pistils or petals, but some other means of propagation,--and so in other cases. Now according to our theory during the infinite number of changes, we might expect that an organ used for a purpose might be used for a different one by his descendant, as must have been the case by our theory with the bat, porpoise, horse, &c., which are descended from one parent. And if it so chanced that traces of the former use and structure of the part should be retained, which is manifestly possible if not probable, then we should have the organs, on which morphology is founded and which instead of being metaphorical becomes plain and <and instead of being> utterly unintelligible becomes simple matter of fact{155}. {155} In the _Origin_, Ed. i. p. 438, vi. p. 602, the author, referring to the expressions used by naturalists in regard to morphology and metamorphosis, says "On my view these terms may be used literally." <_Embryology._> This general unity of type in great groups of organisms (including of course these morphological cases) displays itself in a most striking manner in the stages through which the foetus passes{156}. In early stage, the wing of bat, hoof, hand, paddle are not to be distinguished. At a still earlier <stage> there is no difference between fish, bird, &c. &c. and mammal. It is not that they cannot be distinguished, but the arteries{157} <illegible>. It is not true that one passes through the form of a lower group, though no doubt fish more nearly related to foetal state{158}. {156} See _Origin_, Ed. i. p. 439, vi. p. 605. {157} In the _Origin_, Ed. i. p. 440, vi. p. 606, the author argues that the "loop-like course of the arteries" in the vertebrate embryo has no direct relation to the conditions of existence. {158} The following passages are written across the page:--"They pass through the same phases, but some, generally called the higher groups, are further metamorphosed. ? Degradation and complication? no tendency to perfection. ? Justly argued against Lamarck?" This similarity at the earliest stage is remarkably shown in the course of the arteries which become greatly altered, as foetus advances in life and assumes the widely different course and number which characterize full-grown fish and mammals. How wonderful that in egg, in water or air, or in womb of mother, artery{159} should run in same course. {159} An almost identical passage occurs in the _Origin_, Ed. i. p. 440, vi. p. 606.