CORRELATION OF GROWTH. 177 



When we consider how every activity of living 

 matter has been gradually acquired through constant 

 repetition, each new method of activity or growth 

 being added to and conditioned by that method of 

 growth which already existed, we can understand 

 how this singleness of plan necessarily arose. Every 

 new part and function could only be a modification 

 of that which already existed. Bearing in mind this 

 singleness of plan, and remembering the power of 

 the hereditary impulse in preserving shape, we can 

 attain to an understanding of the phenomena of 

 nearly similar parts performing different functions, 

 in cases where, upon the old theory of use and 

 environment, we might expect as great a difference 

 between the parts as between the functions. For 

 example, if we expected use and function alone to 

 determine the shape and structure of different parts, 

 we could never explain the similarity between the 

 human toes and fingers. For the toes and fingers 

 are very similar in form, in spite of the fact that 

 their functions are widely different. This similarity 

 of form is due originally to the singleness of plan, 

 and secondarily to the strength of the hereditary 

 impulse which has not yet fully succumbed to the 

 influence of the change of function. 



But apparently, the leg and arm being developed 

 from a single co-ordination of forces, the one limb 



