192 THE INDUCTIONS OF BIOLOGY. 



at large. The component cells, thus subject to mechanical 

 disturbances, small and great, perpetual and occasional, are 

 ever having protoplasm forced into them and forced out of 

 them. There are gurgitations and regurgitations which, if 

 they do not constitute a circulation properly so called, at 

 least imply an unceasing redistribution. And the implica- 

 tion is that in the course of days, weeks, months, years, each 

 portion of protoplasm visits every part of the body. 



Without here stating specifically the bearings of these 

 inferences upon the problems of heredity, it will be manifest 

 that certain difficulties they present are in a considerable 

 degree diminished. 



§ 54c. Eeturning from this parenthetical discussion to 

 the subject of structure, we have to observe that besides 

 facts presented by tissues and facts presented by organs, 

 there are certain facts, less general than the one and more 

 general than the other, which must now be noted. In the 

 order of decreasing generality an account of organs should 

 be preceded by an account of systems of organs. Some of 

 these, as the muscular system and the osseous system, are 

 co-extensive with tissues, but others of them are not. The 

 nervous system, for example, contains more than one kind of 

 tissue and is constituted of many different structures : be- 

 sides afferent and efferent nerves there are the ganglia im- 

 mediately controlling the viscera, and there are the spinal 

 and cerebral masses, the last of which is divisible into 

 numerous unlike parts. Then we have the vascular system 

 made up of the heart, arteries, veins, and capillaries. The 

 lymphatic system, too, with its scattered glands and ramify- 

 ing channels has to be named. And then, not forgetting the 

 respiratory system with its ancillary appliances, we have the 

 highly heterogeneous alimentary system; including a great 

 number of variously-constructed organs which work together. 

 On contemplating these systems we see their common 

 character to be that while as wholes they cooperate for the 



