one is led to believe that the tendency to the earlier inher- 

 ence of degenerative modifications producing retrogression is 

 inheritable like the tendency to the earlier inheritance of ad- 

 ditional or novel characteristics producing progression. Thus, 

 this law applied to progressive or retrogressive groups explains 

 the mode in which their progression or retrogression is accom- 

 plished so far as the action of the laws of genesiology (science 

 of heredity) are concerned. 



In the same essay on Bioplastology, the writer reviewed Dr. 

 Minot's law of growth, and in this and in his Phylogeny, 

 quoted above, used it to throw light upon one of the most 

 difficult problems of evolution. 



It is a general law of unique importance, as readilv observ- 

 able in the growth of skeletons and shells of all kinds, and 

 therefore as obvious in fossils as in the famous guinea pigs 

 studied by Dr. Minot. This law enabled the writer to get 

 what seemed to him a clearer view of the action of tachygenesis 

 See Bioplastology (p. 76). 



Minot's researches enable one to see clearly that the reduc- 

 tion of parts or characteristics which takes place through the 

 action of the law known as the law of acceleration in develop- 

 ment (often also descriptively mentioned as abbreviated or 

 concentrated development) cannot be considered as due to 

 growth. 



" It seems probable from my own researches published in 

 various communications, but more especially in the ' Genesis 

 of the Anetidae,' 6 that the action in this case is a mechanical 

 replacenwnt of the earlier and less useful ancestral characteristics 

 and even parts by those that have arisen later in the history of the 

 group. We can fully understand the phenomena of accelera- 

 tion in development only when we begin by assuming that 

 the characteristics last introduced in the history of any type 

 were more suitable to the new conditions of life on the horizon 

 of occurrence of the species than those which characterized the 

 same stock when living on preceding horizons or in less special- 

 ized habitats. These new characters would necessarily, on 



Smithsonian Contributions to Knowledge, v. 26, p. 40-48, 1889- also Mus- 

 Comp. Zool., v. 16, 1889. 



