successive ages continuing to be adapted by selection for some one object, and the larva for quite another one, we need not wonder at the difference becoming wonderfully great between them; even as great as that between the fixed rock-barnacle and its free, crab-like offspring, which is furnished with eyes and well-articulated, locomotive limbs{483}. {481} See p. 42, where the same illustration is used. {482} _Var. under Dom._, Ed. ii. vol. I. p. 452. {483} _Origin_, Ed. i. p. 441, vi. p. 607. _Importance of embryology in classification._ We are now prepared to perceive why the study of embryonic forms is of such acknowledged importance in classification{484}. For we have seen that a variation, supervening at any time, may aid in the modification and adaptation of the full-grown being; but for the modification of the embryo, only the variations which supervene at a very early period can be seized on and perpetuated by selection: hence there will be less power and less tendency (for the structure of the embryo is mostly unimportant) to modify the young: and hence we might expect to find at this period similarities preserved between different groups of species which had been obscured and quite lost in the full-grown animals. I conceive on the view of separate creations it would be impossible to offer any explanation of the affinities of organic beings thus being plainest and of the greatest importance at that period of life when their structure is not adapted to the final part they have to play in the economy of nature. {484} _Origin_, Ed. i. p. 449, vi. p. 617. _Order in time in which the great classes have first appeared._