CORRELATION OF GROWTH. 181 



distinct from each other. Consequently, we might 

 expect that whatever would retard or accelerate the 

 development of one single layer would affect in a 

 similar manner all the parts derived from that layer, 

 i.e. there would be a correlation between all the 

 parts derived from the same germ-layer. This kind 

 of correlation we find between the hair, teeth, and 

 claws of animals, all of which are derived secondarily 

 from the skin and primarily from the epiblast. The 

 single origin of all parts of the muscular system 

 explains why relatively strong muscles in one part 

 of the body are generally accompanied by strong 

 muscles in the other parts. Since we have observed 

 a similar origin for the skin and nervous system, it 

 is interesting to note in man the curious correlation 

 between the colour of the skin, or " complexion," and 

 the mental or nervous temperament. 



The analysis of the correlation of parts helps us 

 to apprehend, if not the method, at least the degree 

 in which the conditions and interacting forces of a 

 developing organism are complicated. 



The specialisation of growth of the tissues goes 

 farther than the primary division into the three 

 germ-layers. Each of these three layers is further 

 divided into a number of secondary tissues, each 

 layer developing only its own peculiar tissues. Any 

 one of these tissues, if partly destroyed, has the 



