334 A HISTORY OF BIRDS 



port of this contention that intra-organismal selection may 

 proceed along lines which, though resulting in evolution, yet 

 appear to have no relation to the struggle for existence, or to 

 the question of the evolution of species. 



Let us take one or two examples. The casque which sur- 

 mounts the head of the Cassowary is composed of a mass of 

 cancellated tissue of great delicacy, ensheathed in horn. Origin- 

 ally, there can be but little doubt, this ossification was purely 

 dermal and rested upon the frontal and nasal bones. In course 

 of its further development the base of this mass of bone fused 

 with the mesethmoid or median partition of the orbits and 

 nasal region. Later, by its backward extension it came to rest 

 upon the frontals. With this relationship an absorption of the 

 frontal so covered gradually took place until, as in the skulls of 

 the Cassowaries of to-day, this absorption has become so com- 

 plete that the hinder end of the base of this casque has actually 

 eaten its way, as its were, into the brain case, so that it now 

 forms a part of the actual brain covering. Thus we have a 

 parallel case to what has happened in the formation of the 

 carapace of the tortoises where what were sometime dermal ossi- 

 fications now take the place of ribs ! 



Similarly, the squamosal bone in birds was originally a 

 purely external bone, overlying the parietal, otic, lateral occi- 

 pital, but in the majority of living birds it has gradually 

 absorbed the underlying osseous tissue and has forced its way, 

 so to speak, into the interior of the skull. Here again is a 

 process of evolution which cannot be supposed to have any 

 influence, or at any rate any conceivable influence, on the ques- 

 tion of the struggle for existence and the origin of species. 



The problem of intra-organismal selection then is one of 

 great difficulty, but it is also of entrancing interest. The struggle 

 between the parts appears to go on irrespective, in a sense, 

 of external conditions and is yet controlled thereby. Intra- 

 organismal selection, in short, suffers no check from external 

 conditions until, and unless, the results of this selection puts the 

 organism as a whole out of harmony with the condition of 

 existence. But it is in this struggle between the parts that we 

 must look for evidence of the origin of species. 



