172 Causes and Course of Orgaxic Evolution 



large group of glucosides existed in plant tissues, and there 

 readily split up, in presence of acids or ferments, into glucose 

 and other products, if such glucose were not utilizable by the 

 plant, at least in times of emergency. But such a relation 

 in the struggle for existence would not at all act in so efficient 

 a manner for continued formation, and selective retention, 

 of accessory products, as when a plant is directly saved from 

 extinction by the presence of a protective substance. 



If a short summary be now given of the contents of the pres- 

 ent chapter it might be said that: (1) a comparative review 

 of caryotic or nucleate plant and animal tissues, from the 

 simple unicellular forms to the most complex multicellular 

 ones, shows that there is direct continuity and uniformity 

 in all the important cell substances, and in all the cell reactions 

 to en\dronal stimuli; (2) that gradual but increasingly funda- 

 mental change in environal relations and stimuli may start 

 the production of a few new substances, as for example cutin, 

 lignin, or inulin in plants, and muscular, cartilaginous, or 

 osseous tissue in animals; (3) that, during the diversified chem- 

 ical actions and reactions involved in the above, accessory 

 products may be formed that may primarily be purely waste 

 substances, as ^dth crystals and hydrocarbons in plants, or 

 some pigments and odorous substances in animals. These 

 may be broken doT\Ti and excreted as is usual in animals, or 

 if helpful as defensive or attractive agents may be conserved 

 and even added to by natural selection, as has largely occurred 

 for sedentary plants; (4) that the appearance and in turn 

 disappearance of some accessory product, or even of essential 

 substances, in places, quantities, and relations that formerly 

 possessed them, results from gradually changed lines of energy- 

 flow, which may again be reeistablished or may continue the 

 new lines of flow with resulting metabolic activity in the new 

 centers involved; (5) that such changes usually start through 

 microscopically small alterations in the size, shape, quantity 

 of substance involved, and energy stimuli of the cell or cells 

 in question, and so start at first imperceptible progressive evolu- 

 tionary changes, as happens to most organisms. Or the altera- 



