28 GENERAL PEELIMINAEY CONSIDEKATIONS. 



they cannot be solved by the few animal forms on which 

 physiology is wont to make her experiments. It is certainly 

 no exaggeration to say that not more than six or eight— at most 

 twelve — VJTifla of animals from among the many thousand 

 existing forms, have hitherto been investigated by physiology 

 as it is understood in our universities. 



But even supposing that the laws of organic physiology had 

 been deduced from the investigation of the greater number of 

 Uving animals instead of merely a few, it still could not avail to 

 answer the questions which arise from the reciprocal relations 

 of animals, and which bear upon the external conditions of 

 existence. But these, above all others, are those that claim our 

 interest, if our point is to establish the most universal laws of 

 the development of organisms and of the transmutation of one 

 form into others. 



In order to set this in a clear Ught, I think it will be 

 advisable to eomparis, in the most general manner, two groups 

 of facts which apparently have no common point of coincidence 

 — the geographical distribution of animals, and the normal 

 arrangement and functions of organs in the individual animal. 



All the organs of an animal are in co-ordination, physio- 

 logically as well as morphologically. Although the liver and 

 blood-vessels, brain and muscles, and all the organs appear to 

 act independently of each other, they are so absolutely depen- 

 dent on each other that they are whoUy incapable of doing 

 their duty as soon as their relations, sometimes very remote, 

 to the other organs are interrupted. Thus the muscles of the 

 arm, though the arm itself were uninjured, would cease to act 

 with any purpose if they were made independent of a healthy 

 will ; and this again depends on the normal activity of our 

 vascular system, for if the blood-vessels of the brain are exces- 

 sively or insufficiently supplied the functional activity of the 

 will must suflfer. And every separate organ is in the same 

 way influenced, and its activity determined, by others, or by all 

 the rest. If one organ is in any degree changed, every other 

 will be affected and changed more or less. 



The same law applies in a certain measure to the present 

 distribution of animals on the surface of our globe. It is no 



