DEGRADATION OF ORGANISATION 69 



local concentration if they are of the first importance, and that finally 

 they are completely and definitely extinguished before the opposite 

 end of the chain is reached. 



As a matter of fact, the degradation of which I speak is not always 

 gradual and regular in its progress, for often some organ disappears 

 or changes abruptly, and these changes sometimes involve it in peculiar 

 shapes not related with any other by recognisable steps. 



Often again some organ disappears and re-appears several times 

 before it is definitely extinguished. But we shall see that this could 

 not have been otherwise ; for the factor which brings about the pro- 

 gressive complexity of organisation must have had varied effects, 

 owing to its liabiUty to modification by a certain other factor acting 

 with great power. We shall however see that the degradation in 

 question is none the less real and progressive, wherever its effects 

 can be seen. 



If the factor which is incessantly working towards compUcating 

 organisation were the only one which had any influence on the shape 

 and organs of animals, the growing complexity of organisation would 

 everywhere be very regular. But it is not ; nature is forced to submit 

 her works to the influence of their environment, and this environment 

 everywhere produces variations in them. This is the special factor 

 which occasionally produces in the course of the degradation that we 

 are about to exemphfy, the often curious deviations that may be 

 observed in the progression. 



We shall attempt to set forth in full both the progressive degra- 

 dation of animal organisation and the cause of the anomalies in the 

 progress of that degradation, in the course of the animal series. 



It is obvious that, if nature had given existence to none but aquatic 

 animals and if all these animals had always lived in the same cUmate, 

 the same kind of water, the same depth, etc., etc., we should then 

 no doubt have found a regular and even continuous gradation in the 

 organisation of these animals. 



But the power of nature is not confined within such limits. 

 It first has to be observed that even in the waters she has estabUshed 

 considerable diversity of conditions : fresh-water, sea water, still or 

 stagnant water, running water, the water of hot cUmates, of cold 

 cUmates, and lastly shallow water and very deep water ; these provide 

 as many special conditions which each act differently on the animals 

 Uving in them. Now the races of animals exposed to any of these 

 conditions have undergone special influences from them and have 

 been varied by them all the while that their complexity of organisation 

 has been advancing. 



After having produced aquatic animals of all ranks and having 



