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PREFACE 



THIS book deals with the subject of evolution in a some- 

 what discursive manner, but its main theme is indicated 

 by the title. It points out that groups of like animals, 

 such as are small in membership and temporary in 

 duration, are common in nature. These groups are not 

 considered to be equivalent to species, they are referred 

 to as family groups. A large part of the book deals 

 with the origin, fate, and significance of such groups. 

 Here and there the question, What is a species ? is 

 raised, but not answered. The tenth chapter is an 

 attempt to show that the Mutation Theory is important 

 to the physician, and the last chapter borders on philo- 

 sophy. The whole appears more compact, if it be regarded 

 as an attempt to help on the new movement in biology, 

 to spread those ideas referred to by some as the Mutation 

 Theory, by others as Mendelism, ideas that are per- 

 manently associated with the names of Mendel, Bateson, 

 and De Vries. 



The writer's warrant for asserting himself lies not in 

 the bulk, but in the variety of his experience, which has 

 been derived from several sources, but from none in 

 particular. 



At the present day so many books are written by 

 specialists for specialists that it is well to point out that 



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