THE LIVING MACHINE BUILDING FACTORS. 183 



members of his race in having exceptionally large 

 arm muscles ; but here, again, the large muscles 

 have been produced by use. A European who 

 has lived under a tropical sun has a darkened 

 skin, but this skin has evidently been darkened 

 by the action of the sun, and is quite a different 

 thing from the dark skin of the dark races of 

 men. In such instances we have variations pro- 

 duced in individuals as the result of outside influ- 

 ences acting upon them. They are not inborn, 

 but are secondarily acquired by each individual. 

 We call them acquired variations. 



It is not always possible to distinguish be- 

 tween these two types of variation. Frequently 

 a character will be found in regard to which it is 

 impossible to determine whether it is congenital 

 or acquired. If a child is born under the tropical 

 sun, how can we tell whether its dark skin was 

 the result of direct action of the sun on its own 

 skin, or was an inheritance from its dark-skinned 

 parents ? We might suppose that this could be 

 answered by taking a similar child, bringing it up 

 away from the tropical sun, and seeing whether 

 his skin remained dark. This would not suffice, 

 however ; for if such a child did then develop a 

 white skin, we could not tell but that this lighter- 

 coloured skin had been produced by the direct 

 bleaching effect of the northern climate upon a 

 skin which otherwise would have been dark. In 

 other words, a conclusive answer can not here be 

 given. It is not our purpose, however, to attempt 

 to distinguish between these two kinds of varia- 

 tions, but simply to recognize that they occur. 



Our next problem must be to search for an ex- 



