160 Causes and Course of Organic Evolution 



animals. Thus the alkaloid xanthin is met with in the blood 

 and in various glands of animals, as well as in some flowering 

 plants. But since animals, by action of and reaction to envir- 

 onal stimuli, by struggle for existence, and other cooperating 

 causes, have in most cases become highly active so as to escape 

 from devourers, have developed efficient defensive scales, 

 plates, shells, secretions, or colors, so as to resist, or be over- 

 looked by, other animals as attackers, and have used these 

 coverings also as protection against over-rapid changes of 

 temperature, light, and other physical environal agents, the 

 result has been that the variety of accessory chemical substances 

 is not nearly so extensive amongst animals as amongst plants. 

 A comparative chemico-biological study of them that ad- 

 mirably supplements the more purely chemical descriptions 

 of the organic chemistry text-book is given in the works of 

 Wiesner, Czapek, Tschirch, and Fischer. Here also the fact 

 cannot be too strongly insisted on that all of these secondary 

 and accessory compounds are as exactly chemical bodies in 

 their composition as are such inorganic substances as chloride 

 of sodium or common salt, carbonate of lime or marble, and 

 sulphate of magnesia or Epsom salts. Like these also they 

 are in nearly all cases crystalloids, and so stand in marked 

 contrast to the carbohydrate, amidated, and albuminous col- 

 loids. But a significant feature is that so far few of them 

 have been built up synthetically, and these few have been 

 obtained only by elaborate chemical or electric laboratory 

 methods. This is another argument we consider in favor of 

 the activity of combined bio-cognitic energies in the tissues. 



The glucosides, as at present known, are vegetable products, 

 and are mainly yielded by the higher flowering plants, since 

 by far the larger number are peculiar to and obtained from 

 the tissues of dicotyledons The exact chemical formula of 

 each has been determined in most cases, and this shows that 

 some like amygdalin (C20H27NO11) may contain nitrogen and 

 be quaternary. But the majority consist of compounds of 

 carbon (C), hydrogen (H), and oxygen (O), that are combined 

 in rather comi)lex proportions. Like the succeeding group 

 they mamly act as protective substances against animal and 



