EXCRETION— REPRODUCTION— IIIIVTIIM 5 



part in various functions of the body, such as growth, and 

 they are also the cause of many obscure diseases. A secretion 

 is a product which serves a useful purpose in the economy 

 of the organism. 



But with excretion we have to deal with the formation of 

 bodies which are useless or injurious to plant or animal. 

 Unless these are discharged from the body, the whole organism 

 gets choked, just as ashes may put out, in time, the driving 

 fire of a steam engine. Plants as a rule store away their excreta 

 in parts of the body where it is harmless, although those plants 

 that cast their bark and shed their leaves get rid of a good deal 

 of excreta annually. Animals excrete sweat, urea, and certain 

 products from the alimentary canal, but the great bulk of the 

 last-named have never formed part of the organism which re- 

 jects them. They have passed through as undigestible portions 

 of the food. In certain animals such as the Ascidian, or sea- 

 squirt, the urea is stored away where it is harmless to the body, 

 but in most animals the urea is taken up from the blood by the 

 kidneys and passed to the exterior. Carbon dioxide (CO.,) is 

 another excretion which passes away from the gills or lungs, 

 and out from the skin like the sweat. A certain amount of 

 excreta is got rid of by Arthropod a (Crustacea, Insecta, 

 etc.) when they cast their skin, and the same is true to a 

 certain extent of some worms. Matter which is not living does 

 not secrete or excrete. 



Living matter grows and reproduces. Animals and plants 

 give rise to successors and they, in their turn, reproduce. 

 The most primitive method of reproduction is that the animal 

 or plant splits or cleaves in two. Each of the two will then 

 increase in size until it reaches a certain bulk — this is groivth 

 — and then again it will divide. No dead or non-hving object 

 behaves in this way. 



Finally, hving matter is rhythmic. It is always doing some- 

 thing or other at stated intervals. These intervals often seem 

 to have no relation to outside influences, like breathing or 

 the recurrent beats of a heart; but in many cases the intervals 

 between the acts correspond with cosmic changes. Niglit and 

 day control sleep; the tides have a marked influence on the 

 habits of many of the shore-living Invertebrates, and so 



