CAUSE OF HEREDITY 291 



only in small degrees through the formation of new tis- 

 sues and mainly by direct remolding of the old material 

 into a new body, having the correct proportion of the 

 species." 



Coming back to the question of variation I quote the 

 following from Mr. Walker: "Variability appears to be 

 a property common to all living organisms in spite of the 

 fact that individual animals and plants produce new in- 

 dividuals that are generally similar to themselves. For 

 instance, if a collie dog be mated with another collie, the 

 pups produced will grow to about the same size as their 

 parent, they will have similarly shaped heads and be 

 similar in general. Every individual in the litter of pups 

 will differ in some way from its brothers and sisters and 

 also from its parents. But though these differences are 

 very evident, upon careful examination they are in the 

 overwhelming majority of instances comparatively small 

 differences. The pups will in fact, though differing from 

 their parents, still resemble them beyond all comparison 

 more nearly than they will resemble a fox-terrior or any 

 other breed of dog." Now this is just what we should 

 expect, that the cells would build the pups as nearly like 

 the parents as they could remember, but we know that 

 in building anything from memory, it is not likely that 

 the structures will be exactly alike. However here is a 

 different proposition stated by Mr. Walker: "The cry- 

 stalline lens of the eye is produced from epiblast but when 

 it is removed in the salamander the new lens grows from 

 cells that were produced from the mesoblast. Under 

 normal conditions mesoblast cells in the salamander 

 would never become modified into anything at all like the 

 crystalline lens. Here the mesoblastic cells have still 

 retained the general potentiality of the fertilized ovum 

 in a very high degree and are able to reproduce such a 



