1926] Hall: Shiill of the Ttodeni Otospermopliilus r/ ram munis heechei/i 399 



habits tile differences that are readily noted between skulls of these 

 two genera may be largely attributed to heredity. Granted that the 

 assumption of identical life-habits by two different stocks of animals, 

 let us say of animals belonging to two different families, results in 

 exactl}^ similar structure throughout, in other words, that a case of 

 complete convergent evolution occurs, the assumption would obviously 

 be difficult of demonstration, since the fact that they were originally 

 derived from different stocks would not be recognizable unless the 

 l)alaeontological and embryological history correlated with present dis- 

 tribution should demonstrate the polyphyletic origin. It seems evi- 

 dent, therefore, that, while the habits of the animal have a very great 

 deal to do with changes that occur in the skull after a certain time, 

 heredity is the primary factor before this time, and that heredity, and 

 environment expressing itself in the habits of the animal, both, are 

 more or less concerned at all times in moulding the developing mam- 

 malian skull. Indeed, it seems doubtful that any character of the 

 mammalian skull, however selected, can be shown to be the result of 

 only one of the two factors, heredity and environment. 



It has already been pointed out that an adequate comprehension of 

 the changes occurring in the skull as a whole involves keeping con- 

 stantly in mind the changes that occur in the component parts. The 

 opportunity for discussion of these changes is therefore practically 

 unlimited. If it were known just which changes are significant, further 

 discussion Avould be profitable. While comparisons of the stages here 

 presented with adult conditions in other forms, especially those 

 believed to be more primitive, might be made to some advantage, it 

 is believed that the full significance of these changes will be recognized 

 only when comparisons are made between the changes in the different 

 forms that are due to growth. It was previously pointed out that 

 differences in the relative amount of growth and the time at which 

 the growth occurs have been noted in different genera. In some 

 groups, probably, the same could be said of species as well as of genera. 

 Parker (1885, p. 41), speaking of three species of Armadillo that he 

 had examined, says: ". . . . Yet they show a most extraordinary 

 amount of variation, and suggest to me, that it would be well worth 

 while to work out the various stages of every species in the family." 

 Therefore, since it is believed that more material of the proper age for 

 comparison is necessary to determine the relative significance of many 

 of the changes noted in Otospermophrlus grammurus heecheyi, it is 

 also believed that the presentation of usable data, that is, data showing 

 the changes in shape and proportion due to different amounts of 



