582 NATURAL SCIENCE. oct., 



the individual, it thereby renders the chemical and physical consti- 

 tution of the stock, upon which the endowment of the individual with 

 those characters depends, advantageous to the stock. The efficiency 

 of the selection depends entirely upon its giving the advantage to 

 those stocks among the new individuals of which the characters 

 advantageous to the individual are numerically. most frequent. All 

 stocks may produce some individuals well-fitted and others ill-fitted 

 for competition in various proportions. Those stocks alone succeed 

 in the competition which produce on the average the greatest pro- 

 portion of well-fitted, the smallest proportion of ill-fitted. That is, 

 the constitution which determines the production of a large proportion 

 of well-fitted new individuals confers an advantage upon the stock 

 possessing it. A constitution which determines undue frequency of 

 variation in a direction disadvantageous to the individual, is not only 

 a disadvantage to the stock, but the disadvantage increases in its 

 efficiency for elimination at every subsequent stage of evolution, till 

 at last it becomes absolutely fatal, and every stock which is endowed 

 with such a constitution is rooted out. Thus is variation in a stock 

 first definitely limited. Stocks not only differ but also vary in fitness ; 

 every unfit stock, every less fit stock, every new stock varying in the 

 direction of less-fitness is weeded out. As the intensity of the 

 struggle increases, this limitation of variation of the stock in unfit 

 directions becomes more and more definite. Thus only those stocks 

 survive in which the constitution is such as to allow of variation only 

 to a very limited extent in certain directions, while in other directions 

 the variation is almost unlimited. 



Such a limitation is not, however, what is ordinarily called 

 heredity ; it is not a simultaneous limitation of variations in all direc- 

 tions ; there is in this supposed stage of evolution none of that 

 thorough similitude among individuals which the continuity school 

 have without justification assumed to have existed ab initio. In any 

 one stock or group of stocks in one definite environment variation is, 

 so far, limited in frequency and extent in certain directions only. 

 The range of variation and its frequency is limited only with reference 

 to certain characters ; but as the struggle for existence increases in 

 intensity, not only is this average range of variation progressively 

 narrowed, but the limitation is gradually extended to characters 

 hitherto varying almost unchecked. Characters hitherto unstable 

 in the extreme are gradually reduced to stability. The number 

 of characters thus rendered comparatively 'constant is gradually 

 increased till it reaches near to infinity. The range of their variation 

 is gradually narrowed till it becomes almost infinitesimal both in 

 extent and in frequency. Genera, if not even species, are now established. 

 This relative constancy introduces a new factor. The whole con- 

 stitution upon which so large a number of almost constant characters 

 depends, must itself have been reduced to almost complete constancy. 

 It is difficult to imagine that a constitution hmited in its variations 



