OTHER THEORIES OF EVOLUTION 331 



Dendy, I.e. ) to disturbances of a physiological nature in 

 the normal developmental processes. It is as well to bear 

 in mind the striking analogies pointed out by Bland Sutton 

 (1890) between such phenomena and pathological growth- 

 phenomena in the individual. On the other hand, Huxley 

 (1932) has sought an explanation in the principle of hetero- 

 genic growth aided by selection, and Haldane has formulated 

 an explanation (1932) of this type of orthogenesis ' on strictly 

 Darwinian lines ' (p. 328). Before examining these theories, 

 however, it is desirable to give examples of the phenomena 

 in question. 



Broadly considered, these examples can be divided into two 

 classes, according to whether (A) exaggerated size of parts 

 or (B) exaggerated complexity is involved. Some structures, 

 however, exhibit excessive size accompanied by exaggerated 

 complexity. Again, both abnormal size and exaggerated 

 complexity are found in sexually dimorphic characters. 



(A) Mammalia : 



Horns of Titanotheria (Osborn, 1929). 

 Canine teeth of Machaerodonts (Loomis, 1905). 

 Antlers of the Irish Elk (Woodward, 1909). 

 Tusks of Elephas ganesa (Lang, 1921) and E.primi- 



genius (Loomis, I.e.). 

 Horns of Water Buffalo (Bos bubalis macrocerus) . 



Reptilia : 



Bony plates of Stegosauria (Loomis, I.e.). 



Mollusca : 



Lower valve of Hippurites and Rudistes (Lang, 



I.e.). 

 Umbonal growth (and flexure) of Ostraea (Lang, 



I.e.). 

 Heavy and elaborately ornamented shells of 



various genera (Lang, I.e.). 



Insecta : 



Foliaceous enlargement of tibia in Anisoscelis 

 (Cuenot, 1925). 



Polyzoa : 



Excessive deposition of CaC0 3 in skeleton of 

 Cretaceous Polyzoa (Lang, I.e.). 



