.^. BIOLOGICAL THEORIES. 581 



nor need it involve the likeness of either of them to the parent mass 

 as a whole. Among the separated portions which, by virtue of their 

 constitutions (chemical and physical), were capable of life and growth, 

 there must have been likeness in all those characters which were, 

 under the conditions then existing, necessary to life and growth. 

 Any separated portions not possessing those characters must have 

 died, and need not be further considered. 



After many generations all individuals living must have agreed 

 not only in possessing all the characters necessary to life, but also in 

 having descended from an unbroken series of ancestors similarly 

 endowed ; and this, not because these characters were of necessity 

 present in all the offspring of all the ancestors, but because all which 

 did not possess them must have failed to live, and therefore to leave 

 descendants. This weeding out of the unfit individuals indepen- 

 dently both of heredity and of competition, and the correlated survival 

 of all the fit and not of the fittest only, is the first stage in the evolu- 

 tion of heredity. Those stocks fail to increase in numbers whose 

 constitution does not determine the production, by some means or other, 

 of new individuals capable of life, growth and multiplication. Within 

 those limits, however, variation may be unchecked. Similarity in 

 form, size, colour, etc., need not exist among the individuals. A large 

 proportion of the offspring may indeed be incapable of life, but those 

 stocks which produce the largest numbers of new individuals capable 

 of life and multiplication will, by virtue thereof, gradually become 

 most numerous. The first stage in the evolution of heredity is the 

 weeding out of stocks whose variation in certain directions renders 

 them incapable of numerical increase. Let this not be misunderstood ; 

 I do not mean that in some stocks there may be a " hereditary 

 tendency to variation " which leads to failure in the struggle for exis- 

 tence ; I do not assume that the offspring are from the first like the 

 parents. I have already denied that the division of a cell into two 

 necessitates the likeness of those two ; even at this stage of evolution 

 the vast majority of new individuals may be incapable of life. 

 The favoured stocks are those which, by virtue of their constitution, 

 however widely it may vary, are capable of producing an increase in 

 the number of individuals, the variation among the individuals 

 being thus far limited only with respect to those characters which are 

 essential to life. 



The second stage in the evolution is marked by the commence- 

 ment of the struggle for existence in the ordinary sense of the term, 

 that is, the beginning of competition : it is the stage in which each indi- 

 vidual comes to be an adverse element in the environment of others. 

 This is the stage when the fit compete against each other, and the 

 fitter alone survive. The competition between individuals is, how- 

 ever, not of immediate interest to us ; it is the struggle between 

 stocks, which is the efficient factor in evolution. As individual com- 

 petition renders certain characters in the individual advantageous to 



