418 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 19 30 



out of account the unicellular organisms which retain their individu- 

 ality only for the few hours or minutes necessary to reorganize and 

 divide again. Even if we assume that the average length of life of 

 an individual is a year — and it is probably not over a day — then we 

 have to consider the remarkable phenomenon of an annual wiping 

 off of the slate, as it were, of this infinitude of individuals each 

 year and their re-formation the next year. This is possible owing to 

 the immense reproductive capacity of certain species. Thus one 

 oyster may lay 50,000,000 eggs during a few days in the summer and 

 one sea urchin 20,000,000 eggs. These are samples, merely, of repro- 

 ductive capacity of individuals. Perhaps now we have gained some 

 conception of the number of individuals that have been produced 

 each year on the earth, during we do not know how many millions 

 of years. 



If now you are willing to admit that the problem of organic evo- 

 lution is that of the evolution of an organic mass consisting of an 

 infinitude of individuals reproduced during an infinitude of genera- 

 tions, that may serve as a starting point to our inquiry as to the 

 mechanism of organic evolution. 



Now, each individual has a certain recognizable form and acquires 

 it through a certain course of development, be it more or less com- 

 plex. The center of control of this form is largely, if not chiefly, in 

 the chromosomes of the organic cells — in its genes, to be more pre- 

 cise. In fact the soma of organisms, what we see, is just an index 

 of the form producing and maintaining factors of the genes — al- 

 ways, of course, recognizing that the end result is a sort of re- 

 action between gene and environment. Now if the world of organ- 

 isms is composed of an infinitude of kinds it is because the germ 

 plasm is of an infinitude of kinds. The course of organic evolution 

 has been and is what it is because the germ plasm has undergone 

 and is undergoing the changes that it has undergone and is under- 

 going. This change of the germ plasm is called mutation. Muta- 

 tion is one of the great factors in organic evolution. 



Now what do we know about mutation? First, we know that it 

 is widespread. This Imowleclge has first become precise as organ- 

 isfils have been studied in successive generations, under controlled 

 conditions. Such mutations have long been known among domesti- 

 cated organisms like potatoes, poultry, guinea pigs, and dogs. Muta- 

 tions have been so long known among domesticated organisms that 

 it was natural for Darwin to discuss " Variation under Domestica- 

 tion " and for him and others to consider what quality of domesti- 

 cation it is that induces mutation. During the past 25 years in 

 several species of animals taken from the wild, many generations 

 have been followed. And in consequence we now know that muta- 

 tion has no necessary relations to domestication; but only that 



