302 ELEMENTS OF BIOLOGY 



Chapter I, namely, that all living organisms endeavor to avoid 

 death, either actively by the functioning of the property of irri- 

 tability and by escape, or passively by reason of its powers of adap- 

 tation to environmental circumstances. Protoplasm was described 

 as a complex colloidal system in which the laws of physics and 

 chemistry operate; but we know of no non-living colloidal system 

 that either actively or passively changes character to avoid coagula- 

 tion or other disintegrative influence. 



Cycles. So far as the life of any particular animal is concerned, 

 death closes and extinguishes its existence as living matter. But the 

 species is perpetuated through the rejuvenescence of the zygote 

 during reproduction and much of the energy expended during indi- 

 vidual life is devoted to this continuation of the species. Thus the 

 continuity of the species is all important; the life of the individual 

 is of secondary importance. With the fertilization of the egg a new 

 individual is initiated and continues as the carrier of life to suc- 

 ceeding generations. From the general picture thus presented one 

 obtains the view that the path of an animal species through time is 

 a succession of cycles; tgg, development, adult, egg, development, 

 and adult succeeding each other as the centuries pass. One also 

 thinks of an individual animal in terms of its life cycle and notes 

 that in some species a life cycle may include several varieties of 

 form. For example, the hydroids exhibit an alternation of sexual and 

 asexual generations; various parasitic worms assume several forms 

 within the life cycle (p. 260). 



Cyclic successions characterize many phases of animal life. We 

 are already familiar with the cycles of nitrogen and of carbon in 

 Nature. Cells pass through division cycles. Individuals pass through 

 reproductive cycles and life cycles. Is it possible that the species of 

 animal life with which we are familiar are passing through species 

 cycles, that they are destined to be succeeded by other species in 

 the future? 



