726 General and Applied Biology 



of one or more minor hereditary factors, (2) fluctuations in environment, 

 or (3) the interaction of the above two causes. 



According to Their Utility. — Variations may be such that they are 

 harmful, indifferent, or useful to the organism possessing them. The 

 utiHty is determined not only by the degree of variation but also by the 

 type of environment in which the organism is asked to use that type of 

 variation. What is useful in one environment might be harmful in 

 another. 



WrJst 



Wrist- 



Fig. 360. — Comparative stages in the elevation of the horse's foot to the tip of 

 the middle toe, as shown by the human hand. (Courtesy of The American 

 Museum of Natural History.) 



CAUSES OF VARIATIONS 



The origins of variations are known as ( 1 ) germinal if they arise in- 

 ternally in the germ plasm, and (2) somatic if they arise in the soma- 

 toplasm (body plasm) due to external environmental causes, either dur- 

 ing embryologic development or in adult life. 



Charles Darwin considered variations as "axiomatic" or self-evident 

 and thus needed no explanation. Lamarck regarded the causes of varia- 

 tions as being ( 1 ) intrinsic or physiologic, in which case the organism put 

 forth an internal eflfort or response to successfully adapt itself to its 

 particular surroundings or (2) extrinsic or external, in which the ex- 

 ternal environmental factors produced or caused the variations. In the 



