RELATIONS OF THE THREE KINGDOMS. 3 



crystal is by additions to the outside, layer by layer ; 

 the growth of the living thing is by material taken into 

 every part of each cell — interstitial growth. 2. Again, 

 the growth of the crystal is by materials exactly like 

 itself already existing in the liquid, not made by the 

 crystal ; whereas the living thing makes the material of 

 its growth ; it manufactures material like itself out of mate- 

 rial wholly different from itself, and then uses it for growth 

 — growth by assimilation. 



4- Life History. — All living things pass through a 

 regular cycle of changes determined by forces within 

 itself. They are born, increase, culminate, decay, and 

 finally die. No such cycle of changes is observed in 

 nonliving things. Whatever changes they undergo are 

 accidental and the result of external causes. 



5. Reproduction. — As living things have a definite 

 term of existence, they must reproduce their kind ; 

 otherwise the organic kingdom would speedily pass 

 out of existence. This also is characteristic of living 

 things. 



6. Waste and Supply — Metabolism.— Continual 

 change of the material composing the body in every 

 part is very characteristic of living things. The living 

 body has been compared to a whirlpool : the matter is 

 continually changing while the form remains. In the 

 living body the change is in some sense self-determined. 

 This ceaseless internal change by waste and supply is 

 called metabolism. It is rapid in proportion as the life 

 is high. 



We are concerned here only with living things. All 

 dead things and the sciences which concern them are 

 therefore put aside; but we must still limit our field. 

 We are not concerned with all living things, but only 

 with animals. We must therefore distinguish between 

 animals and plants. 



