HEREDITY. 401 



sive increment or evolution from earlier or later geo- 

 logic periods. There are two possible explanations 

 of this phenomenon. The one is that the characters 

 of one generation are inherited by the next, which 

 adds to them by the activity of the same stimuli which 

 gave them origin, thus producing progressive increase 

 of growth. The alternative is, that these structural 

 characters are produced by each generation for itself. 

 It is obvious that the latter hypothesis provides for 

 no additional development of a character in one gen- 

 eration above another. There are other objections to 

 the latter view, but letting these pass for the present, 

 it is only necessary to examine the embryonic history 

 of animals to show that it is entirely untenable. For 

 if some or all of these acquired characters can be 

 found present in the early stages of growth, as in the 

 egg, the pupa, the foetus, etc., it becomes clear that 

 such acquired characters have been inherited. That 

 such is the fact is abundantly demonstrated by embryo- 

 logical researches. This fact alone is sufficient to set 

 at rest by an affirmative answer the question as to the 

 inheritance of acquired characters. And that this 

 answer applies to all time and to all evolution is made 

 evident by the fact, which is disclosed by paleontology, 

 that all characters now congenital have been at some per- 

 iod or another acquired. 



2. EVIDENCE FROM EMBRYOLOGY. 

 a. Vertebrata. 



I have already (p. 292) pointed out the gradual 

 evolution through mechanical causes of the tongue and 

 groove-joints in the Mammalia as exhibited by the 

 distal ends of the metapodial bones of the feet where 



