XI. EVOLUTION AND MUTATION , 403 



from the Asiatic continent probably ever since the Cretaceous period, 

 and nowhere else in the world do we find so many old-fashioned relics 

 of earlier mammalian forms. At the time of the Australian region 

 was cut off from the continent of Asia, marsupials were probably the 

 highest mammals in the region. In their isolation they were not 

 brought into competition with the higher types of mammals which were 

 evolving on the other continents ; hence they have lingered along the 

 path of development at the lower levels of mammalianism. Evolution, 

 however, has not been at a standstill among them, for we find that 

 special types have arisen to occupy many of the available physical and 

 nutritional stations of animal existence. Thus, there are carnivorous 

 forms such as the so-called Tasmanian wolf, and herbivorous forms 

 like the kangaroos ; or in terms of dwelling place there are land forms 

 (Tasmanian wolf, kangaroo, wallaby), burrowing forms (bandicoots, 

 marsupial mole), arboreal forms (phalangers. Koala, Dasyurus) and vol- 

 planing forms (flying phalangers). 



It should be noted that these forms of divergency, commonly called 

 adaptive radiation, are very similar to those of other continents where 

 various higher types of mammals have been developed, indicating that 

 the direction of the evolution of mammals is strikingly restricted. 

 Presumably this is due to the highly developed mechanism by which 

 environmental influences are reduced to limited forms, and the mech- 

 anism may consist of the hormones. 



Organisms especially higher animals appear to be able to avoid 

 abrupt mutations by such elaborate means. It may be said, therefore, 

 that organisms liable to undergo mutations are only defective specimens, 

 never fitted to continue their existence. 



2. Genie Changes due to Chemical Agents 



Typical, gradual changes in higher animals are considered to be 

 induced by physical environmental factors, such as climatic ones, which 

 are transferred to the genes after greatly modified by hormones, 

 whereas chemical agents appear to affect the genes directly and ab- 

 ruptly ; accordingly, the change due to chemical agents is rather a 

 mutation, occurring rapidly and tending to be irreversible. 



Viruses afford a good example of the mutation of this category. 

 The specific structure of viruses is transmitted directly to the genes, 

 and the change occurs rather rapidly, tending to be irreversible and 

 usually even injurious to the organism. The action of pneumococcus- 

 transforming principle, as discussed repeatedly, may belong to this 

 category. This principle causes no injurious effect unlike usual viruses. 



