16^ Causes and Course of Organic Evolution 



activity tlian do the algoid or moss alliances. They are occa- 

 sional in the fern alliance and the gymnospernis, about twenty 

 occur amongst the higher monocotyledons, and the remain- 

 der amongst dicotyledons. Further, they are almost absent 

 amongst members of the Incompletse, are fairly abundant 

 and varied amongst the apopetalous groups, but are most 

 frequent in the sympetalous or highest dicotyledons. 



Present facts would lead us to consider that they have been 

 evolved as accessory or by-products, that have by natural 

 selection remained in the plant tissues, to act there for de- 

 fensive protection, or have again been largely or wholly ab- 

 sorbed. But that this action is limited in its range ecologically 

 is proved by the capacity which various fungi possess of causing 

 widespread havoc amongst coffee, tea, and similar crops, and 

 which either haA^e become immune to definite alkaloids or 

 have formed an antitoxin in their tissues that neutralizes the 

 alkaloidal poison of the host. The condensed account given 

 by Czapek (2Ii., 2: 927-28), regarding this line of inquiry, 

 should be consulted. 



The volatile oils and resins form a very large group of com- 

 pounds, that are shortly distinguished from the two previous 

 groups by being insoluble in water. They are generally con- 

 tained in special cells or intercellular spaces of plant tissues. 

 Their average chemical composition shows that they are simpler 

 than the alkaloids or glucossides, but functionally they act 

 like these as defensive substances. They are rare amongst 

 the simpler plants up to and including the fern alliance, are 

 widely distributed, though not varied in number, amongst the 

 Coniferse, and are specially characteristic of higher dicoty- 

 ledons. Some plant families like the Pinacese or Coniferse, 

 the Aurantiacese, Myrtacese, and Lamiacese are specially 

 rich in them, and so resist fungoid and animal attack well. 

 But the micro-fungi, and the boring insects that ramify through 

 the tissues of some of the above, again prove that inununity 

 against destructive environal agents has been a prolonged and 

 fierce struggle on the part of surviving members of the sedentary 

 plant world. 



Very few volatile secretions have been met mth amongst 

 the lower animals, but amongst mammals they are frequent, 

 castoreum of the beavers being one example. The tail, foot, 

 facial, and other glands of different mammals all yield volatile 

 products. 



The tannins and tannin conipounds with approximate formula 

 C14H10O4 can next be reviewed, for like the three last groups 

 they have the same approximate distribution in the plant 



