The Evidence from Intellectual Differences. 245 



parent, or in any more remote ancestor. The slight in- 

 dividual differences are so overshadowed by the much 

 more conspicuous resemblances due to heredity with 

 which they compare about as the green buds at the tips 

 of the twigs of a large tree compare with the hard wood 

 of the trunk and branches, the growth of previous years 

 and they are so fluctuating and inconstant, that their 

 importance may easily escape attention. Careful obser- 

 vation shows, however, that every characteristic may 

 vary: those distinctive of the class or order as well as 

 those which mark the species or variety. The variations 

 may manifest themselves in the adult, or at any other 

 period in the life of the individual. Even the eggs have 

 individualities of their own, and among many groups of 

 animals the eggs of the same parent, when placed under 

 precisely similar conditions, may differ in the rate and 

 manner of development. Although most of these indi- 

 vidual differences are transient, and disappear within a 

 few generations, there can now be no doubt that those 

 which tend to bring the organism into more perfect har- 

 mony with its environment, and are therefore advantage- 

 ous, may be established as hereditary features, through 

 the action of the law of the survival of the fittest; and it 

 is hardly possible to over-estimate the value of the evi- 

 dence which paleontology and embryology now furnish to 

 prove that all hereditary characteristics, even the most 

 fundamental, were originally individual variations. 



The series of hereditary structures and functions 

 which makes up the life of an organism is constantly be- 

 ing extended by the addition of new features, which, at 

 first mere individual variations, are gradually built into 

 the hereditary life history. In this way newly acquired 

 peculiarities are gradually pushed further and further 

 from what may be called the growing end of the series, 



