334 ENVIEONMENT AMONG ANIMALS AND PLANTS 



nature of these differences, it is probable that in part at least 

 these differences are environmental. Such observations help to 

 exhibit the degree of importance that we are led to attribute 

 to modifications among free-living animals in a state of nature. 

 It must also be borne in mind that departures from the normal 

 range of variations of the environmental stimuli are not so very- 

 infrequent and that more or less extreme modifications, as result, 

 for instance, from the attacks of parasites, do occur. 



Lastly we may go a step farther and ask what happens when 

 the environment changes. This inquiry, if pursued, would lead 

 us beyond the scope of this chapter. A reference, however, to 

 this problem may assist to render the relation of a species under 

 natural conditions to its environment rather more clear. Change 

 of environment may be due either to the migration of some or 

 all the members of a species to a locality where the environment 

 is different, or to an actual change in the environment in the 

 same locality. If the change is at all marked, then there will 

 be a different response on the part of the germinal constitution 

 to the new conditions. Let us suppose that the diet is changed and 

 that some members of a species of bird take to a diet of grain 

 having previously existed on a diet of fish. There will be a different 

 response in that the stomach will assume a different form. But it is 

 most unlikely that the old germinal constitution will be that which 

 gives the best response to the new conditions. Almost certainly 

 a somewhat different type of germinal constitution will be that 

 which will give the best response to the new conditions, will be 

 that, in other words, from which there will arise the form of 

 stomach most suited to dealing with grain. If and when, there- 

 fore, a mutation arises exhibiting this changed type of germinal 

 constitution, it will be favoured, and in this manner a new variety 

 and ultimately a new species may be formed. It has thus to be 

 remembered that, although the germinal constitution to some 

 extent responds differently to different stimuli, it is very unlikely 

 that any average type of germinal constitution will give the best 

 response to any other environment than that to which the species 

 is now subject. It does not therefore follow that, because the 

 responses of the germinal constitution are various, there are not 

 factors making for a change in the germinal constitution when 

 the environment changes. 



Summing up what we have said, we have to think of every 



