378 THE POPULAR SCIENCE MONTHLY 



This interpretation broadens, I think, our general conception of 

 natural selection. We see that it is erroneous to suppose that all the 

 individuals that bear a particular, useful trait owe this trait to their 

 descent from one kind of individual, in the sense that this individual is 

 the sole ancestor of all the later survivors. The first individual is not 

 alone the ancestor of all the individuals that later bear its mark, it is 

 only one of 999,999 ancestors that have contributed to the perpetuation 

 of the race. 



In order to simplify the case we have imagined that the new varia- 

 tion has appeared in a single individual. Should it appear in more 

 than one, or arise again and again, its implantation would be thereby 

 hastened, but the principle remains the same. 



My contention may be summed up in a sentence. Survival- value is 

 not the only test for the perpetuation of any one useful character; it 

 is the sum total of useful variations that determines progress. The 

 species moves as a group always. Evolution is not a simple but a com- 

 plex problem. This is the general opinion held by most modern 

 zoologists. 



To-day there are three great rival claims that attempt to explain 

 how evolution takes place : ( 1 ) that which adopts the theory of natural 

 selection in one or another of its aspects; (2) that which maintains that 

 acquired charact^s are inherited; (3) that which, trying to pene- 

 trate deeper into the mystery of life, ascribes to living matter a purpose- 

 fulness — an almost conscious response to " the course of nature." 



In a few concluding words I shall try to point out the standing of 

 these rival claims. 



Darwin himself adopted both the first and the second of these veiws. 

 His whole philosophy stands opposed in principle to the third view. 

 He did not hesitate at times to adopt the theory of the inheritance of 

 acquired characters, whenever the facts seemed more in accordance 

 with that interpretation than with that of natural selection. He 

 strenuously objected that he had never intended to refer the entire 

 process of evolution to natural selection, and later in life affirmed that 

 he had perhaps laid too little stress on the influence of the environment. 

 To-day the doctrine of inherited effects is in disgrace, largely owing to 

 the brilliant attack of the philosopher of Freiburg. Nevertheless it has 

 warm adherents; and not a few of the most cautious zoologists now 

 living have expressed themselves in its favor. It has not lacked able 

 advocates, but it has sadly lacked direct evidence to support it. I can 

 show you an example of how it fails when put to the test. I have here 

 a waltzing mouse that turns round in circles instead of moving for- 

 wards. This is a domesticated variety and breeds true, i. e., all of its 

 offspring are waltzers. I next show you a pair of mice that were in- 

 jected with acetyl-atoxyl to cure them of sleeping sickness. They 

 have artificially acquired the same habit as a result of the injection, and 



