398 University of California FutUcations in Zoologii [Vol. 21 



the reason for this complete early development of the nervous system 

 is a physiological one, there is still a. utilitarian cause back of it, and 

 this need of reaction to the environment almost immediately after birth 

 is suggested as possibly the cause. Whether or not the problem of use 

 and disuse be taken into account, any consideration which attempts to 

 explain the relatively large size of the postorbital region of the skull at 

 birth should take into account the fact that the brain is phylogenetic- 

 ally an older structure than the skull and has undergone no degenera- 

 tion as the skull has developed. 



The greatest amount of increase occurs in the preorbital region of 

 the skull. A relatively well developed state of this region would seem 

 to be less necessary to the animal at the time of birth than would a 

 relatively well developed state in certain other regions for the reason 

 that the preorbital region is in a great degree concerned with the intake 

 of solid food ; this is a function performed first at a time much later 

 than the time of birth. The time at which the preorbital region of 

 the skull undergoes the greatest amount of increase is seen to be 

 between E and J. This is the period during which the animal changes 

 from a diet of milk to one of solid food. 



In general, I have become more and more impressed with the belief, 

 which is, I think, generally held, that, until birth and for a short time 

 thereafter, the greatest single factor in shaping the skull is the brain. 

 The sensory capsules also exert a profound influence upon the embryo 

 skull. In skulls slightly 3'ounger than A, the intruded auditory cap- 

 sules occupy an extremeh^ large area, as is the case in most vertebrates. 

 The brain appears to be the main determining factor of shape of the 

 skull before birth, but seems to be superseded, shortly after birth, by 

 muscular development. Indirectly, the changes in shape after birth 

 appear to be in a large measure the results of the habits of the animal 

 in so far as habits are expressed in relations of bones and muscles of 

 the skull, through the mechanical strains exerted by muscles upon the 

 bones of the skull. From comparisons of skulls of various ages of 

 different kinds of mammals, I conclude that food habits are, indirectly, 

 chiefly responsible for the changes in shape that begin to occur shortly 

 after birth, and thus responsible, also, for the shape of the adult skull. 

 This explanation of the causes for the ultimate shape of the skull cer- 

 tainly derives some support from the similarities found, for example, 

 between the skulls of Castor and Erethizon, although, in these two 

 animals, the food habits are not identical, merely similar. So far as 

 I am aware certain members of the genus Dipus and the genus Bipo- 

 domys have very similar habits. If the two do have exactly similai- 



