On the Origin of Species · ¶673

In certain cases the successive steps of variation might supervene, from causes of which we are wholly ignorant, at a very early period of life, or each step might be inherited at an earlier period than that at which it first appeared. In either case (as with the short-faced tumbler) the young or embryo would closely resemble the mature parent-form. We have seen that this is the rule of development in certain whole groups of animals, as with cuttle-fish and spiders, and with a few members of the great class of insects, as with Aphis. With respect to the final cause of the young in these cases not undergoing any metamorphosis, or closely resembling their parents from their earliest age, we can see that this would result from the two following contingencies; firstly, from the young, during a course of modification carried on for many generations, having to provide for their own wants at a…

Charles Darwin · John Murray, London, 1859, first edition · Gutenberg #1228; original diagram from Darwin Online F373
Charles Darwin · John Murray, London, 1859, first edition · Gutenberg #1228; original diagram from Darwin Online F373
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