GEOGRAPHICAL DISTRIBUTION. 139 



temperature, depth, pressure, medium, and, in general, con- 

 ditions of environment in which they are distributed. And 

 at the same time we learn (a) that this is the case for a small 

 group of organisms whose general structure is alike, all secret- 

 ing a spiral shell, all having substantially the same organs, 

 arranged in much the same manner, and (b] that the range of 

 the differences of environment concerned viz., in tempera- 

 ture, in depth, from the abysses to the tops of mountains, in 

 mediums, from high-pressure salt water to rarefied air is 

 almost as complete as it would be possible to reach in habit- 

 able regions of the globe. The differences in form and 

 structure of the organisms as units are, therefore, not at 

 all in proportion to the differences of conditions of environ- 

 ment. Organisms very much alike, in the same genus even, 

 are found living under conditions of environment as strongly 

 contrasted, almost, as can be found ; and organisms of extreme 

 difference in structure are associated together in the same 

 conditions of environment. The conclusion we draw is, that 

 condition of environment is a fundamental cause in determin- 

 ing differences of form, but that whatever the structure or 

 organization of an organism may be, there have been, and are 

 constantly going on, adjustments to changed habitats, and that 

 the morphological changes resulting in these adjustments to 

 environment have been mainly of low order, i.e., varietal or 

 specific, and rarely are of higher than generic importance. 



This is in strong constrast to the law observed regarding 

 relation of differences of form to time ; amount of time-sepa- 

 ration being co-ordinate with degree of difference in the 

 whole structure, and not merely in specific and generic 

 characters. 



Summary. In what has been said above the relations of 

 form to general conditions of environment have been dis- 

 cussed. Geographical distribution, in the particular use of 

 the term, is concerned with the association of like forms (the 

 same species or varieties) in areas presenting like conditions 

 of environment, and the distinguishing of different areas by 

 the different faunas and floras inhabiting them. It is sup- 

 posed that the adjustment, by various processes, of the species 

 to their changed environment may explain their differences 



