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 variation 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 complex 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 characters are inherited; (3) that which, trying to penetrate deeper into the mystery of life, ascribes to living matter a purposefulness—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. 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 forwards. 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 injected with acetyl-atoxyl to cure them of sleeping sickness. They have artificially acquired the same habit as a result of the injection, and