12 HEREDITY AND INHERITANCE 



being transmissible, among multicellular organisms reproducing 

 sexually, is extremely doubtful. 



Evolution. — Briefly and concretely stated, the general doc- 

 trine of organic evolution suggests, as we all know, that the 

 plants and animals now around us are the results of natural 

 processes of growth and change working throughout unthinkably 

 long ages ; that the forms we see are the lineal descendants of 

 ancestors on the whole somewhat simpler ; that these are de- 

 scended from yet simpler forms, and so on, backwards, till we 

 lose our clue in the unknown, but doubtless momentous, vital 

 events of pre-Cambrian ages, or, in other words, in the thick 

 mist of life's beginnings. The essentially simple idea is that 

 the present is the child of the past, and the parent of the future. 

 It is a way of looking at organic history, a genetic description, 

 a modal formulation. A process of Becoming leads to a new 

 phase of Being ; the study of evolution is a study of Werden 

 mid Vergehen mid Wetter- werden. 



But we have to pass from a modal interpretation to a causal 

 one. We have to try to discover the factors in the age-long 

 process, and this leads us into a region where at present uncer- 

 tainties abound. As biologists we start with the postulate of 

 simple living organisms — feeding, working, growing, wasting, 

 reproducing in an appropriate environment. And we try to 

 discover the possible factors in the long evolution-process, the 

 outcome of which is the present-day world of life. Amid all 

 the uncertainties, this is certain, that the fundamental condition 

 of evolution is that genetic relation which we call heredity, — a 

 relation such that it admits, on the one hand, of a continuity 

 of hereditary resemblance from generation to generation ; and, 

 on the other hand, of an organic changefulness which we call 

 variability. Without the hereditary relation there could have 

 been no succession of generations at all. Without hereditary 

 resemblance on the one hand, and hereditary variation on the 

 other, there could have been no evolution. Any discussion of 



