Ill EFFECTS OF CHANGES IN THE ENVIRONMENT 83 



difference in the organisation of the animal (as distinct from that 

 of the shell alone) is necessary for the permanent constitution of 

 specific rank.^ What amount of structural difference is required, 

 what particular organ or organs must exhibit this difference, will 

 depend largely upon the idiosyncrasy of the observer. But if 

 this, or something like this definition of a species be accepted, 

 it will follow that a so-called 'variety' Avill be a form which 

 exhibits differences from the type which do not amount to per- 

 manent structural differences in the organisation of the animal. 

 The final court of appeal as to what affords sufficient evidence 

 for 'permanent structural differences' will have to be, as with 

 Aristotle of old, the judgment of the educated man. 



It is, however, more to our present purpose to discuss the 

 causes of variation than to lay down definitions of what variation 

 is. One of the most obvious causes of variation lies in a change 

 or changes in the environment. If we may assume, for the 

 moment, that the type form of a species is the form wdiich is the 

 mean of all the extremes, and that this form is the resultant of 

 all the varied forces brought to bear upon it, whether of food, 

 climate, temperature, competition, of numbers, soil, light, amount 

 of water, etc., it will follow that any change in one or more of 

 these forces, if continuous and considerable, any change, in other 

 words, of the environment, will produce its effect upon the 

 organism in question. And this effect will be for the better or 

 for the worse, according to the particular nature of the change 

 itself as tending towards, or away from, the optimum of environ- 

 ment for the species concerned. Hence may be produced vari- 

 eties, more or less marked according to the gravity of the change, 

 although it must be noted that at times a change apparently 

 unimportant from our point of view, will produce very marked 

 results upon the species. It is indeed scarcely possible to predict 

 with any certainty, in the present state of our knowledge (beyond 

 certain broad results) what will be the particular effect upon a 

 species of any given change in its surroundings. 



Effects of Change in the Environment as tending to 

 produce Variation. 



(a) Changes in Climate^ Temperature^ Elevation^ etc. — In the 

 eastern basin of the Baltic the marine Mollusca are much more 



1 J. W. Taylor, Journ. of Conch, v. p. 289, an interesting article, with many- 

 useful references. 



