It was not so long ago that we were taught that the instincts of animals are the inherited experience of their ancestors—lapsed intelligence was the current phrase.
Lamarck's name is always associated with the application of the theory of the inheritance of acquired characters. Darwin fully endorsed this view and made use of it as an explanation in all of his writings about animals. Today the theory has few followers amongst trained investigators, but it still has a popular vogue that is widespread and vociferous.
To Weismann more than to any other single individual should be ascribed the disfavor into which this view has fallen. In a series of brilliant essays he laid bare the inadequacy of the supposed evidence on which the inheritance of
acquired characters rested. Your neighbor's cat, for instance, has a short tail, and it is said that it had its tail pinched off by a closing door. In its litter of kittens one or more is found without a tail. Your neighbor believes that here is a case of cause and effect. He may even have known that the mother and grandmother of the cat had natural tails. But it has been found that short tail is a dominant character; therefore, until we know who was the father of the short-tailed kittens the accident to its mother and the normal condition of her maternal ancestry is not to the point.
Weismann appealed to common sense. He made few experiments to disprove Lamarck's hypothesis. True, he cut off the tails of some mice for a few generations but got no tailless offspring and while he gives no exact measurements with coefficients of error he did not observe that the tails of the descendants had shortened one whit. The combs of fighting cocks and the tails of certain breeds of sheep have been cropped for many generations and the practice continues today, because their tails are still long. While in Lamarck's time there
was no evidence opposed to his ingenious theory, based as it was on an appeal to the acknowledged facts of improvement that take place in the organs of an individual through their own functioning (a fact that is as obvious and remarkable today as in the time of Lamarck), yet now there is evidence as to whether the effects of use and disuse are inherited, and this evidence is not in accord with Lamarck's doctrine.
THE UNFOLDING PRINCIPLE
Nägeli and Bateson
I have ventured to put down as one of the four great historical explanations, under the heading of the unfolding principle, a conception that has taken protean forms. At one extreme it is little more than a mystic sentiment to the effect that evolution is the result of an inner driving force or principle which goes under many names such as Bildungstrieb, nisus formativus, vital force, and orthogenesis. Evolutionary thought is replete with variants of this idea, often naïvely expressed, sometimes unconsciously implied. Evolution once meant, in
fact, an unfolding of what pre-existed in the egg, and the term still carries with it something of its original significance.