26 CHARLES R. STOCKARD 



as in the dogs, the genetic history is associated with devia- 

 tions in endocrine constitution and considerable change may 

 result from a single point mutation. Yet just as the modified 

 achondroplasic head of the prize bulldog is a character de- 

 pending upon the homozygous condition of several genetic 

 factors for its complete expression, as we shall indicate in 

 a subsequent chapter, so the flippers of the seal with their 

 achondroplasic-like skeleton, modified muscular arrangements 

 and fin-like metamorphosis of the foot probably originated 

 from an initial mutation causing the loss of spinal neurones 

 and a subsequent series of mutations associated with the 

 specialization of the limb. 



In two giant breeds of dogs there is the tendency to pro- 

 duce individuals with a lameness resulting from degeneration 

 following paralysis of the thigh muscles. The condition varies 

 in its expression depending upon the homozygosity of the 

 more or less complete complex of probably three dominant 

 mutant genes. The more nearly homozygous the individual 

 becomes for all three of these genes, the more pronounced 

 and extensive is the degeneration. Possibly the completely 

 homozygous complex might give highly modified lower ex- 

 tremities, though such an individual has probably not yet 

 been bred. All the thigh muscles are not paralyzed and quite 

 definite ones persist and compensate in such ways as to give 

 hyperextension and eversion of the leg, producing a gait 

 suggesting closely the peculiar ambling of the seal. The in j 

 heritance of this paralysis may in some way be genetically 

 associated with the inheritance of skin overgrowth and certain 

 acromegalic features which are further associated with a 

 modified pituitary reaction, since all the breeds involved show 

 these characters. It is of further interest to note that most 

 of the aquatic mammals have thickened and wrinkled over- 

 growth of skin, as well as heaviness of the anterior body 

 regions and a tendency towards the accumulation of fat. The 

 head, neck and shoulders of the paralyzed dogs are large, 

 strongly muscled and covered with excessively loose rolls of 

 skin, while the rump and thighs of such animals are wilted 



