As we shall have to consider the important principle of isolation more fully on a subsequent occasion, I need not deal with it in the present connexion, further than to remark that in this principle we have what appears to me a full and adequate condition to the rise and continuance of specific characters which need not necessarily be adaptive characters. And, when we come to consider the facts of isolation more closely, we shall find superabundant evidence of this having actually been the case. V. _Laws of Growth._ Under this general term Darwin included the operation of all unknown causes internal to organisms leading to modifications of form or structure--such modifications, therefore, appearing to arise, as he says "spontaneously," or without reference to utility. That he attributed no small importance to the operation of these principles is evident from the last edition of the _Origin of Species_. But as these "laws of growth" refer to causes confessedly unknown, I will not occupy space by discussing this division of our subject--further than to observe that, as we shall subsequently see, many of the facts which fall under it are so irreconcilably adverse to the Wallacean doctrine of specific characters as universally adaptive, that in the face of them Mr. Wallace himself appears at times to abandon his doctrine _in toto_. CHAPTER IX. CHARACTERS AS ADAPTIVE AND SPECIFIC (_continued_). It must have appeared strange that hitherto I should have failed to distinguish between "true species" and merely "climatic varieties." But it will conduce to clearness of discussion if we consider our subject point by point. Therefore, having now given a fair statement of the facts of climatic variation, I propose to deal with their theoretical implications--especially as regards the distinction which naturalists are in the habit of drawing between them and so-called true species.