146 PROBLEMS OF EVOLUTION 



greater superiority to heat and cold. Evolution, in fact, has 

 been a struggle among competing groups ; physical conditions 

 have led to an advance only when they have increased in rigour. 

 This being so, the environment of a species is mainly the species 

 with which it has established a rapport. Now in a previous 

 section (p. 103) I have shown how the competing groups advance 

 part passu. Thus the Peregrine Falcons and their environment 

 have been developed side by side during the slow process of 

 evolution. There have been endless mutual adaptations and now 

 all the competitors stand, each on the top of the huge pile it has 

 thus accumulated. For each the great mass of adaptation is old- 

 established. For each, heredity decrees that there shall only be 

 variation within a certain range unless indeed reversion set in 

 and that would at once be checked by the keen competition. 

 And something more definite still there is the principle of 

 sequence in variation : among the offspring of parents, in which 

 a particular variation has occurred and been protected by 

 isolation, some are likely to vary in the same direction. This 

 principle will be at work among the environing species and the 

 environed, if we like to distinguish them thus in thought, produc- 

 ing further the parallel lines. 



We can now see that there are two limitations to the field 

 within which chance has free play : (l) heredity limits the range 

 of variation for each species ; sequence of variation extending 

 the principle further. (2) The environment also undergoes evolution ; 

 in its case there are corresponding limitations. This is obvious when 

 we bear in mind that when a number of species are en rapport 

 with one another each is part of the environment of all the rest. 

 In this way we can explain the happy coincidences that arise 

 when the rapport between a species and any part of its environ- 

 ment has been once established. But how does the rapport arise 

 Several in the first instance ? This too admits of explanation. There 

 ^vh-on 6 are ^ mes m tne evolution of many species when no great nicety 

 ments of coincidence is necessary, because there is a choice of environ- 

 ments, with any of which a connection may be established, and 

 often there is a variety of possible adaptations to one and the 

 same environment. For example, many plants must have means 



