426 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 



genus the same kind of mutations occur. Thus between DrosophUa 

 melanog aster ^ D. simulans, and D. virilis quite parallel mutations 

 occur and are identified in many cases with genes occupying com- 

 parable loci in the chromosomes. Among mammals, with which we 

 are naturally best acquainted, certain mutations occur again and 

 again. Thus albinism, coat-color pattern, hairlessness, elongated 

 hair, reversed hair, taillessness, abnormal hands and feet, particu- 

 larly digits, horns or hornlessness, achondroplasia. The recurrence 

 of these mutations in various species of mammals, as tabulated by 

 Osborn (1912), indicates that mutations are far from haphazard 

 in origin, but probably depend upon the same genes with the same 

 structure and capacity for change. 



Finally, the mutations with which we are familiar constitute only 

 a fraction of those that occur. Every student of intrauterine stages 

 of mammalian development is familiar with the phenomenon of in- 

 trauterine deaths. So far as our observations go, it appears that in 

 mammals more young die in utero than are born. Every student of 

 development of sea urchins and other marine organisms knows that 

 a large proportion of those that start to develop do not proceed far. 

 We see that early developmental stages are those in which great 

 selection takes place; probably because the embryos carry non- 

 viable mutations. From this point of view the individuals that 

 reach maturity constitute the fraction that have undergone no 

 lethal mutation. 



The fact of lethal mutations (for they have been demonstrated in 

 many cases) helps us to understand the other fact that in the midst 

 of the world of mutations the organisms that come through are more 

 or less well fitted to survive; they are not always the best, but they 

 are good enough to pass the censorship of environment. 



And this brings us to a consideration of the phenomenon which 

 Darwin stressed, namely, of adaptation to environment. We have 

 already seen that such adaptation is partly brought about through the 

 selection by the organism of an environment that is adapted to it. 

 We may now consider the case of selection by the environment of 

 organisms that are adapted to it. In general, the organism must be 

 able to play properly its part in the flow, in and out, of the chemical 

 agents, water and foodstuffs, upon which its life and activity de- 

 pend. It must be capable of meeting emergencies of climate and 

 organic enemies. It must pass the censor at every stage or be 

 squelched. 



A little experience of my own, which I published some years 

 ago, will serve to show how strict is this censor. I reared a large 

 number of chickens to the 1-pound stage and had about 300 running 

 over a grassy plot on about the tenth of May, at a time when crows 

 are feeding their nestlings and hunting especially meat for them. 



