ENVIRONMENT AFFECTS EVOLUTION. 285 



ment, as illustrated in the case of the twins Arthur 

 and John, it will be said, no doubt, that the cause 

 of their equal proportional development is heredity; 

 and this may be true. The question may, then, be 

 raised, What is heredity ? If proportional growth, 

 and a definite series of movements be due to 

 heredity, we have in such series a direct expression 

 of heredity in a measurable form. 



A child is growing like one or other parent in 

 features (trophic combinations) and in manner of 

 walking, speaking, or writing, etc. It is the series 

 of trophic and kinetic actions that express the 

 heredity. This seems easier to admit with regard 

 to trophic than with regard to kinetic series. It 

 seems more in accord with common observation 

 that a certain mode of growth should follow on 

 given external conditions than that a certain series 

 of movements should succeed. 



In the growth of children, on the average, the 

 proportion of growth of limbs and parts of the 

 body, is similar in different subjects at the same 

 age. Is this owing to the impress made upon the 

 embryo (or its germ) and upon its ancestors ? It is 

 certain that afferent forces may alter this, increase 

 size of head, or hands, etc. 



In the plant the variations of proportional 

 growth are easily affected by light (internodes) 

 and by heat. 



In the case of the twin brothers, the reader will, 

 from general knowledge, probably be inclined to 

 accept the view that Arthur and John are of equal 

 proportional development owing to heredity, owing 



