THE PLANT AND ANIMAL COMPARED. 165 



or energy unused (in urea, etc.) ; the latter as excretions, which 

 diffuse freely outwards through the skin, nephridial surfaces, 

 alimentary canal, etc. This is shown in the table on p. 164. 



Of the daily outgo the water, carbon dioxide, and salts are 

 devoid of energy, but the urea contains a small amount which is 

 a sheer loss to the animal. Were the earthworm a perfect ma- 

 chine it could use this residue of energy by decomposing the urea 

 into simpler compounds [viz., ammonia (NH 3 ), carbon dioxide 

 (CO 2 ), and water (H 2 O)] ; but it lacks this power, though there are 

 certain organisms (Bacteria) which are able to utilize the last 

 traces of energy in urea. To the daily outgo must be added the 

 occasional loss, both of matter and of energy, suffered in giving 

 rise to ova and spermatozoa, and in providing a certain amount of 

 food and protection for the coming generation. 



Comparison of the Fern and Earthworm. To the superficial 

 observer the fern and earthworm seem to have little or noth- 

 ing in common, except that both are what we call alive. But 

 whoever has studied the preceding pages must have perceived 

 beneath manifold differences of detail a fundamental likeness 

 between the plant and animal, not only in the substantial 

 identity of the living matter in the two, but also in the 

 construction of their bodies and in the processes by which 

 they come into existence. Each arises from a single cell 

 which is the result of the union of two differently constituted 

 cells, male and female. In both, the primary cell multiplies and 

 forms a mass of cells, at first nearly similar, but afterwards differ- 

 entiated ia various directions to enable them to perform different 

 functions, i.e., to effect a physiological division of labor. In 

 both, the tissues thus provided are associated more or less 

 closely into distinct organs and systems, among which the various 

 operations of the body are distributed. And in both the ulti- 

 mate goal of individual existence is the production of germ-cells 

 which form the starting-point of a new and similar cycle. 



This fundamental likeness extends also to most of the actions 

 (physiology) of the two organisms. Both possess the power of 

 adapting themselves to the environments in which they live. 

 Both take in various forms of matter and energy from the en- 

 vironment, build them up into their own living substance, ar.d 

 finally break down this substance more or less completely into 

 simpler compounds by processes of internal combustion, setting 



