Page:The Effect of External Influences upon Development.djvu/23

Rh by the strength and nature of the influences that affect them; and this capacity to respond conformably to functional stimulus must be regarded as the means which make possible the maintenance of a harmonious co-adaptation of parts in the course of the phyletic metamorphosis of a species. Herbert Spencer has given it as his opinion that in the harmonious working together of parts a cogent reason is to be found for accepting the doctrine of the transmission of acquired characters: but in so doing he has overlooked the fact that there is a never-resting principle at work which is uninterruptedly concerned with the production of harmony, alike in respect of size and functional activity, among parts that co-operate: I mean the principle of intra-selection.

Naturally the degree of discord among the parts may sometimes be such that intra-selection is not able to produce harmony; for there must be definite limits to the scope of adaptation, and we well know that the exercise of a function for too long a time or too violently ceases to produce strengthening of the organ, and causes weakening instead. But as the primary variations in the phyletic metamorphosis occurred little by little, the secondary adaptations would probably as a rule be able to keep pace with them. Time would thus be gained till, in the course of generations, by constant selection of those germs the primary constituents of which are best suited to one another, the greatest possible degree of harmony may be reached, and consequently a definitive metamorphosis of the species involving all the parts of the individual may occur.