History of the Theory of Heredity. 29 



opment of each egg into its proper organism, and if it is 

 true that the egg which is to give rise to a man differs in no 

 essential point from that which is to give rise to an insect, 

 we may conclude that the mystery is too great to be fath- 

 omed by our intelligence, and we may fairly ask what 

 possible explanation can, on this hypothesis, be given of 

 the wonderful properties of the egg. 



The answer which has been given, and which seems 

 to have been thought satisfactory by many students, is 

 this: 



We know, from a mass of evidence which is constantly 

 and rapidly increasing , and to which each new observa- 

 tion adds cumulative weight, that the various forms of 

 life have been slowly evolved, during long ages, from 

 older and simpler forms; that as we trace back the his- 

 tory of any two animals or plants we find evidence that 

 in the past they had for a common ancestor a species 

 which had not yet acquired the distinctive features of 

 either of them; that a little farther back we trace this 

 species to an ancestor with still wider relationships. 



Every day the evidence grows stronger to show that 

 more complete knowledge will ultimately prove that the 

 same thing is true of still larger groups ; that families, 

 classes and orders of organisms have been formed in the 

 same way by gradual modification and divergence ; that 

 complete knowledge of the ancestry of any organ- 

 ism would lead us back through simpler and simpler 

 forms to a remote unspecialized unicellular ancestral 

 form. It is unnecessary to review in this place the evi- 

 dence for this conclusion, for the fact that it is fully ac- 

 cepted by those best qualified to judge of its truth, is 

 perfectly familiar to all students. 



Now it is said, and the explanation is pretty generally 

 accepted, that since any particular organism, a horse for 



