[Chap. XXXIX MUTATIONS 487 



some behavior, and the transmission of hereditary factors during the 

 hfe cycles of plants. 



For all these advances in knowledge civilization is indebted to the 

 scientific method of procedure in experimentation and observation. It is 

 by this procedure that man has learned what to believe about his en- 

 vironment, and how to manipulate a portion of it to his own advantage. 

 In the absence of data, he observes the obvious phenomena about him 

 and speculates about their origin. Dependable decisions come only 

 through intelhgent experimentation and observation, and by repeatedly 

 subjecting the inferences drawn to the test of new situations and addi- 

 tional data. The discovery of basic principles ("laws") of nature enable 

 us to infer with greater precision what has occurred in the past and 

 what is most likely to occur in the future. We can be reasonably sure, for 

 instance, that every person in the world older than we are, was once an 

 infant who developed from a fertilized egg. We can be equally certain 

 that if there are new species of oak trees a million years from now, they 

 will have been derived from oak species now living. The number of such 

 inferences one may draw about living organisms depends upon the num- 

 ber and kind of scientific principles which he fully appreciates. 



You may remember that Camerarius and other investigators during 

 the 18th century fully confirmed the fact that sexual reproduction occurs 

 in plants. But the discovery that fertilization is the actual union of sperm 

 and egg, and that the embryo develops from a fertilized egg, is the 

 product of the experiments of the second half of the 19th century. All 

 sorts of fantastic ideas about sex had prevailed before that time. Each of 

 the other major ideas discussed in these chapters passed through a simi- 

 lar history. We need, therefore, to distinguish clearly these early guesses, 

 which have been passed on to most of us by tradition, from the conclu- 

 sions that may be drawn today from more extensive and more critical 

 data. 



Darwin did not know how to evaluate the various ideas current in his 

 time concerning evolution. He therefore began to collect facts that might 

 help him reach some reasonable conclusions. At the end of 20 years of 

 investigation he concluded that species are mutable, that by variation 

 new species originate from preexisting ones, and that by innumerable 

 slight variations the plants and animals of the present have descended 

 from only a few primordial progenitors of early geologic time. The nu- 

 merous discoveries since Darwin's time have reasonably confirmed his 

 conclusions that species are mutable, and that the plants and animals of 



