DEGRADATION OF ORGANISATION 83 



When afterwards she succeeded in producing the nervous system, 

 it then immediately became possible to create the muscular system. 

 Thereupon, fixed points for the attachments of the muscles became 

 necessary, and also paired parts so as to constitute a symmetrical 

 shape. Hence have resulted various schemes of organisation due to 

 the environment and to the parts acquired, which could not previously 

 have come about. 



When finally she secured sufficient movement in the contained 

 fluids of the animal to permit a circulation to be organised, there 

 again resulted important pecuharities of organisation which dis- 

 tinguished it from the organic systems in which there is no 

 circulation. 



In order to perceive the truth of what I have stated and to furnish 

 evidence of the degradation and simphfication of organisation (since 

 we are following the order of nature in the inverse direction) let us 

 rapidly run through the various classes of invertebrate animals. 



MOLLUSCS. 



Soft unjointed animals which breathe by gills and have a mantle. No 

 ganglionic longitudinal cord; no spinal cord. 



The fifth rank, as we descend the graduated scale of the animal 

 series, necessarily belongs to the molluscs ; for they have to be placed 

 a stage lower than the fishes since they have no vertebral column, 

 but they are yet the most highly organised of invertebrate animals. 

 They breathe by gills, which vary greatly not only in their shape and 

 size, but in their position within or without the animal according 

 to the genera, and the habits of the races comprised in these genera. 

 They all have a brain ; nerves without nodes, that is to say, without 

 a row of gangha stretching down a longitudinal cord. They have 

 arteries and veins and one or several single-chambered hearts. They 

 are the only known animals which, although possessing a nervous 

 system, have neither a spinal cord, nor a ganglionic longitudinal 



cord. 



Gills, which are essentially intended by nature to carry out re- 

 spiration during immersion in the water, have been subjected to modi- 

 fication both in function and shape in those aquatic animals which 

 have been constantly exposed for generations to contact with the air, 

 and even in some cases have stayed in it altogether. 



The respiratory organ of these animals has imperceptibly become 

 accustomed to the air ; and this is no mere supposition : for it is known 

 that all the crustaceans have gills and yet there are crabs {Cancer 



