THEORIES AND MECHANISM OF EVOLUTION 663 



lustrated in Australia where the ancient Under the guiding hand of man a prodi- 

 marsupial stock gave rise to a variety of ani- gious variety of animals have been produced 

 mals that closely parallel the placental to satisfy his whims, whether it be for food, 

 mammals on other continents. For example, as in the case of chickens and cattle, or for 

 there are shrew-like marsupials, squirrel- entertainment, as in the case of canaries 

 like marsupials, dog-like marsupials and and racing horses. He has done a notable 

 many others. It appears that, given only job in the chicken, for example, which, 

 marsupials to begin with and no carnivores with variants from the cocky Bantam to the 

 to destroy them, divergence has been com- tremendous Brahmin, is a far cry from the 

 mensurate with the possible niches, paral- Oriental jungle fowl from which present- 

 leling similar situations where placental day chickens are largely or wholly de- 

 mammals were the starting point. There scended. This ancestor laid perhaps two 

 are many similar cases of divergent and dozen eggs per year, whereas some present- 

 convergent evolution among animals that day breeds equal that number in a month, 

 could be cited. They confirm one of the The possibility of selection toward an eo-o- 

 fundamental features of evolution, namely, factory such as this seems to be limited only 

 that all opportunities are exploited with the by the number of feeding hours in a day 

 materials at hand. How far the diversifica- and the ability of enzymes to transform the 

 tion goes depends on the animal and its food into egg protein and carbohydrates, 

 supply of mutations, as well as the number Certainly this is comparable to selection in 

 of environments in which the animal can nature which in the distant past produced 

 survive. such huge animals as the dinosaurs. As 



these great beasts disappeared because 



EVOLUTION THROUGH evolution proceeded in one direction for 



EXPERIMENTATION greater and greater size, so the domestic 



chicken would soon become extinct if sud- 



Under natural conditions, animal hybrid- denly removed from the protection of do- 



ization probably is of little significance in mestication and set free in a natural envi- 



evolution, although there are cases in which ronment. It seems that progressing on such 



new species of plants have apparently arisen a one-way path spells doom for any species, 



as a result of such natural crossing. Domes- for this has been demonstrated frequently 



tication among animals was carefully stud- in the past. An animal that retains a sort of 



ied by Darwin and he used the results of "middle-of-the-road" type of development 



these studies as a potent argument in favor is more apt to weather the adversities of 



of natural selection. He observed, as we all change and maintain itself as a species, 



have, that under the influence of selective Man is a good illustration in point, 



breeding a wide variety of animals can be Domesticated animals seem to preserve 



produced which, if such forms appeared in their interfertility, quite contrary to condi- 



nature, certainly would be assigned differ- tions found in nature. The reason why they 



ent species names. Darwin substituted en- do is that man so plans it. Sterile animals 



vironment for man as the selective agent are of no use to him, so he has selected for 



and argued that new species could thus high fertility along with other traits that he 



come into being. Since the environment wishes to accentuate. Even with some of his 



"■ was not endowed with reasoning powers, domestic animals the beginnings of infer- 



the tendency would be to select in many tility have been evident. For example, some 



different directions with the ultimate goal crosses between dogs are all but impossible, 



always the adaptation to the particular en- as between a very large one and a very 



vironment in which the animal found itself, small one. Furthermore, some crosses often 



