440 THE GRAMMAR OF SCIENCE 



of a unless there be a selective death-rate. The tendency 

 will be for the type to change progressively, even if the 

 type never reaches the goal a. Before it reaches this 

 goal the change of type may have brought a selective 

 death-rate into play. Precisely the same arguments 

 apply to the second case, except that in this case in- 

 dividuals with more of the character than a will aerain 

 have less reproductivity than those in the neighbourhood 

 of a, thus the distribution will again change in both type 

 and variability, and the type will not become fixed until 

 it coincides with a. We accordingly see that both the 

 first and second cases tend to become identical with the 

 third, in which we have the maximum fertility associated 

 with the type. 



Now consider the third case. Here the modal in- 

 dividuals being most fertile will tend in the absence of 

 selection to become more and more numerous, i.e. the 

 variability will be reduced. Will this reduction go on 

 indefinitely ? No, because even the offspring of a single 

 individual are variable, and there would still be substantial 

 variability in a race, if all its members were really off- 

 spring of type individuals. Thus the goal indicated in 

 all three cases is the same, and we may sum up as 

 follows : — 



If fertility be correlated with any organ or character 

 in any form of life, then : The organism cannot be stable, 

 but must progressively change until the type becomes 

 identical with the character or organ associated with 

 maximum fertility, and until the variability becomes that 

 of the offspring of the type only. 



I do not assert that this result is ever reached, but I 

 do assert that progressive changes in living forms can 

 only be looked upon as the product of the action and 

 reaction of natural selection and reproductive selection. 

 Any novel environment which changes the character a 

 of maximum fertility will also change the type c, even 

 if there be no selective death-rate at all. Any novel 

 environment which introduces a selective death-rate, or 

 changes a periodic selective death-rate into a secular 



