xil INTBOBUOTION 



animals of a given grade, even those which through 

 degeneration have reverted to this grade. 



Even if I cannot trace the exact path which leads to 

 the mountain-top, I may almost with certainty affirm 

 that it leads from meadow and pasture through forest 

 to bare rock, and thence over snow and ice to the sum- 

 mit ; for each of these forms a zone encircling the 

 mountain. Very similarly I find that, whatever gene- 

 alogical tree I adopt, one sequence in the dominance 

 of functions characterizes them all ; digestion is domi- 

 nant before locomotion and locomotion before thought. 



And it is hardly less than a physiological necessity 

 that it should be so. The plant can and does exist, 

 living almost purely for digestion and reproduction, 

 and the same is true of the lowest and most primitive 

 animals. A muscular system cannot develop and do 

 its work until some sort of a digestive system has 

 arisen to furnish nutriment, any more than a steam- 

 engine can run without fuel. And a brain is of no use 

 until muscle and sense-organs have appeared. 



This sequence of dominant functions,* of physiologi- 

 cal dynasties, would seem therefore to be a fact. And 

 our series of forms described in the second, third, and 

 fourth chapters is merely a concrete illustration show- 

 ing how this sequence may have been evolved. The 

 substitution of other terms in the anatomical series 

 there described — amoeba, volvox, etc. — would not affect 

 this result. By a change in the form of our history 

 we have eliminated to a large extent the sources of 

 uncertainty and error. And the dominant function of 

 a group throws no little light on the details of its 

 anatomy. 



* See condeueed Chart of Development, etc., p. 309. 



