il^ ZOOGENESIS '^ll 



If the animal world is fundamentally but a single 

 unit, definite evidence of that fact should be available. 

 We find the evidence in the ability of each and every 

 type of animal, no matter w^hat its structure or its 

 form, to maintain itself equally well in the face of 

 widely varying, though always ruthless, competition. 

 Further evidence of the fundamental unity of animal 

 life is seen in the recurrence of similar forms in crea- 

 tures of widely different structure whenever because 

 of a difference in size, or for other reasons, there is no 

 direct competition, and also in the frequent occurrence 

 in so-called abnormal or aberrant individuals of fea- 

 tures normally characteristic of animals of a widely 

 different type. More tangibly we find it in the recur- 

 rence of comparable and similar attributes in widely 

 different types of life when they are faced with similar 

 conditions. There is no other possible explanation 

 for the reappearance of such striking similarities as 

 those which are found in insects, in the small birds, 

 in the rodents, and in man. 



Mutations. — The third question to be answered is, 

 how do animals change their form? The three factors 

 immediately involved in the production of new animal 

 species or types are; first, the production of variants 

 or mutants; second, the hereditability of the characters 

 possessed by these mutants; and third, the ability of 

 such mutants as are able to establish themselves on an 

 hereditary basis to maintain and to perpetuate them- 

 selves under the conditions they must meet. 



Most mutations arise during the formation of the 

 germ cells, and they are therefore already present in 



