18 DEGENERATION : I 



the adult dog. The changes which have taken count- 

 less ages in the one case, are accomplished in a few 

 weeks in the other. 



And now we have to note the 

 important fact which makes this 

 process of development so intensely 

 interesting in relation to the pedi- 

 FIG. i. An egg : a smgie gree of the animal kingdom. There 



corpuscle of protoplasm IT 



with nucleus b c, and is very strong reason to believe 

 that it is a general law of trans- 

 mission or inheritance, that structural characteristics 

 appear in the growth of a young organism in the order 

 in which those characteristics have been acquired by 

 its ancestors. At first the egg of a dog represents 

 (imperfectly, it is true) in form and structure the 

 earliest ancestors of the dog ; a few days later it has 

 the form and structure of somewhat later ancestors ; 

 later still the embryo dog resembles less remote 

 ancestors ; until at last it reaches the degree of 

 elaboration proper to its immediate forefathers. 



Accordingly the phases of development or growth 

 of the young are a brief recapitulation of the phases of 

 form through which the ancestors of the young creature 

 have passed. In some animals this recapitulation is 

 more, in others it is less complete. Sometimes the 

 changes are hurried through and disguised, but we 

 find here and there in these histories of growth from 

 the egg most valuable assistance in the attempt to 

 reconstruct the genealogical tree. The history of the 

 development of the common frog is a good illustration 

 of the kind of evidence in question. 



