408 METAPHYSICAL EVOLUTION. 



zooid and ovum will partake of tlie character of the memory thus 

 created. The only reason why the oft-repeated stories of birth- 

 marks are so often untrue is because the effect of temporary im- 

 pressions on the mother is not strong enough to counterbalance 

 the molecular structure established by impressions oftener re- 

 peated throughout much longer periods of time. 



The demonstration of the truth or falsity of this position, so as 

 to constitute it the true doctrine of evolution, could only be veri- 

 fied from the prosecution of the science of paleontology. It is 

 only in this field that the consecutive series of structures can be 

 obtained which show the directions in which modification has 

 taken place, and thus furnish evidence as to the causes of change. 

 The most complete result of these investigations, up to the present 

 time, has been the obtaining of sufficiently full series of the Mam- 

 malia of the Tertiary period to show their lines of descent. In 

 this way the series of modifications of their teeth and feet has 

 been discovered, and the homologies of their parts been ascer- 

 tained.* Perhaps the most important result of these investiga- 

 tions is the following : The variations from which natural selec- 

 tion has derived the persistent types of life have not been general 

 or even very extensive. They have been in a limited number of 

 directions, f and the most of these have been toward the increase 

 in perfection of some machine. They bear the impress of the 

 presence of an adequate originating cause, directed to a special 

 end. Some of the lines struck out have been apparently inade- 

 quate to cope with their environment, and have been discon- 

 tinued. Others have been more successful, and have remained, 

 and attained further modification. 



The reader can estimate the chance of the production of an 

 especially adaptive mechanism in the absence of any pressure of 

 force directing growth to that end. It appears to me that the 

 probability of such variation appearing under such circumstances 

 is very slight indeed, and its continuance through many geologic 

 ages directed to the perfecting of one and the same machine still 

 smaller. For this reason, attempts have been made to demon- 

 strate a mechanical cause for the modifications of structure ob- 



* " Homologies and Origin of the Molar Teeth of the Mammalia educabilia." 

 Journal Academy Nat. Sciences, Philadelphia, March, 1874. Proceedings Academy 

 Nat. Sci., 1865, p. 22. 



f See Hyatt on this point, " Tertiary Planorbis of Steinheim." Annir. Mem. 

 Bost. Soc. Nat. Hist., 1880, p. 20. 



