352 Comparative Anatomy — Its History, Aim, and Method 



(1834-1914) emphasized the "continuity of the germ-plasm" from 

 generation to generation and proposed that variations are initiated by 

 varying combinations of the highly complex germ-plasms of male and 

 female parent, with the possibility of direct action of environment on 

 germ-plasm. In 1901 de Vries, in Holland, announced his Mutation 

 Theory, which asserted that new species originate in relatively large 

 abrupt variations. Whether or not the form so produced shall persist 

 may then depend on "natural selection." Bergson, French philos- 

 opher, proposed that the progressive crealiveness of evolution is the 

 expression of an internally acting nonmechanical factor, a "vital im- 

 pulse" (elan vital). 



About 1900, de Vries and other European botanists rediscovered 

 "Mendel's Law of Inheritance," lost to science for nearly 40 years. 

 With this highly important addition to the existing assemblage of 

 theories, the stage was well set for the next act in which leading roles 

 were taken by Biometry which applied mathematics to biologic data 

 and undertook statistical studies of organisms, especially their varia- 

 tions, and by Genetics which, using every available technique of 

 experimentation and of cytology, attacked the problem of the proto- 

 plasmic mechanism of inheritance. It is highly significant of the period 

 that in 1903 the Carnegie Institution of Washington organized its 

 Department of Experimental Evolution, and in 1904 established, on 

 Long Island, New York, its research station with a permanent staff of 

 investigators. Meanwhile it became more and more clearly recognized 

 that, whatever the causal factors in evolution, they must in one way or 

 another involve relations between the physical and chemical constitu- 

 tion of living substance and the physical-chemical complex of the 

 environment. Therefore it was necessary, on the one hand, to learn as 

 much as possible about the chemistry and physical structure of proto- 

 plasm and, on the other, to find out how experimental modification of 

 the physical-chemical environment will affect protoplasm and its 

 characteristic activities such as growth and cell-division. Also the 

 embryologists turned to experimentation. Development is a process of 

 continual change and elaboration of form. When we see substance 

 changing its form, we want to know what mechanical agency — that is, 

 what actual "push" or "pull" present at the instant — is effecting the 

 change of form. Therefore experimental modification of the immediate 

 environment of the embryo should reveal to what extent and in what 

 way development depends upon external physical and chemical factors. 

 It early appeared that, within certain limits, development is internally 

 controlled. Therefore embryos have been subjected to a great variety 

 of operative procedures — removal, addition, interchanges of parts — 



