receive much the same kind and quantity of stimulation, direct and indirect, from the environment, and are possessed of much the same power of varying in response to stimulation, as the structures of the other individuals of the species; as a result corresponding structures in different individuals in corresponding stages of the ontogeny closely resemble one another not only in general form, but also in size. Now since the structures develop mainly in response to stimulation, their development in the later stages of the ontogeny, at any rate, can only be regarded as acquired, not inborn: whence, if it be true that acquired variations are not transmissible, it follows that the later development cannot be transmitted; which is in accordance with the facts observed, for in each individual that later development must be acquired anew in response to stimulation, and cannot be achieved without it. That which is transmitted is the power to vary in response to stimulation, and it is inconceivable that this power can have resulted from aught but the accumulation of inborn variations.
It follows, moreover, that the adult individuals of any higher species differ among themselves as regards their structures, mainly through differences in this inborn power to vary in response to stimulation. Their structures are similar, but they do not develop to the same extent, or in the same proportion. Individuals differ in size mainly because there is inborn in some or all the structures of the bigger individuals, a greater power to vary in response to stimulation, than there is inborn in the structures of the smaller individuals; or, to a much less extent, they differ in size because the structures of the bigger individuals have received more stimulation than the structures of the smaller individuals: they differ in shape mainly because of differences in the power the structures had to vary
