such modifications as are known to occur within the individual life at any time from conception to death or within a single generation. This will throw out of the question a hoard of cases where it is not quite clear whether the observed changes are the result of direct modification, in the true restricted sense, or are perhaps due in part to accumulated influences acting through several generations.
Confining ourselves then to a discussion of modification in the strict sense of the term, let us briefly survey the conclusions of experimental zoology, embryology and botany, and also experiments on regeneration, to see what lesson may be learned from the results of all this painstaking work. It will be of course impossible to treat of more than a fraction of the multitudinous results which have already been handed in as contributions to this young and rapidly growing branch of knowledge—experimental biology; but I believe that even a superficial survey will suffice to bring out my general contention.
The fact that ordinary differences in human environment, as shown in the history of royalty, appeared to alter the innate character and capacity for achievement but little, together with the fact that experimentally a great deal in the way of modification could be produced in the domains of zoology and botany, led me to suppose that there must be some inherent biological differences rendering tissues low in the organic scale especially susceptible to the molding of external influences.
This idea I advanced as an hypothesis in 1906 in the following words:[1]
Among plants and the lower forms of animals, especially the invertebrates, many experiments have shown the remarkable changes which may be directly induced by changes in the outward conditions of life. These are in general the more striking the lower we go in the scale of organic evolution, so that it may well be that in the highest attributes, namely, mental or moral, we can expect the least results from outward forces. This hypothesis may prove a veritable generalization throughout the animal series.
In 1908[2] I expressed the same idea in the following passage:
The profound modifications that may be induced in plants and the invertebrates by alterations of the surroundings are well known. But when one takes a survey of this whole question of modification, one sees that in general the lower we go in the scale of life the easier it is to effect the changes. It is significant that among vertebrates the modifications are largely associated with the integument, where cell-division is active and constant, and in a tissue that is not highly organized.
It is to offer the proof for this generalization that I now propose a survey of the whole question.
