Evidently if osseous structures are produced by deposits of calcareous matters in pre-existing cartilaginous structures, or other structures of flexible materials, the deposits must be so carried on that while dense resistant masses are produced these must admit of such free movements as the creature’s life necessitates, and must so form adapted joints. Let it be understood, however, that the hypothesis set forth in the foregoing chapter and extended by Prof. Cope, which serves to interpret a large part of the phenomena of osseous structures in the _Vertebrata_, does not serve to interpret them all. While the formation of hard parts has been in large measure initiated and regulated by tensions and pressures, there are hard parts the formation of which cannot be thus explained. The bones of the skull are the most obvious instances. These are apparently referable to no other cause than the survival of the fittest--the survival of individual animals in which greater density of the brain-covering yielded better protection against external injuries. Without enumerating other instances which might be given, it will suffice to recognize the truth that natural selection of favourable variations and the inheritance of functionally-produced changes have all along co-operated: each of them in some cases acting alone, but in other cases both acting together.] CHAPTER XVI. THE SHAPES OF ANIMAL CELLS. § 260. Among animals as among plants, the laws of morphological differentiation must be conformed to by the morphological units, as well as by the larger parts and by the wholes formed of them. It remains here to point out that the conformity is traceable where the conditions are simple. [Illustration: Fig. 294.]