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Evidently this relation cannot be accounted for on the assumption of an absolute disappearance; something must remain from which the old features may be restored. This lengthy discussion may be closed by the citation of the cases, in which plants not only show developmental features of a former state, but also reproduce the special features they formerly had, but seemingly have lost.
Having regard to this general scheme of geological knowledge and thought, it is obvious that geological speculation may be, so to speak, anatomical and developmental speculation, so far as it relates to points of stratigraphical arrangement which are out of reach of direct observation; or, it may be physiological speculation so far as it relates to undetermined problems relative to the activities of the earth; or, it may be distributional speculation, if it deals with modifications of the earth's place in space; or, finally, it will be aetiological speculation if it attempts to deduce the history of the world, as a whole, from the known properties of the matter of the earth, in the conditions in which the earth has been placed.
Von Baer taught us, half a century ago, that, in the course of their development, allied animals put on at first, the characters of the greater groups to which they belong, and, by degrees, assume those which restrict them within the limits of their family, genus, and species; and he proved, at the same time, that no developmental stage of a higher animal is precisely similar to the adult condition of any lower animal.
A facsimile of his drawing is here. He says that they were possessed of a rapidly moving lash, and that there were other forms without tails, which he assumed were developmental stages of the form. This is nothing less than the monad whose life-history I gave you last. My drawings, magnified 2,500 diams., of the active organism and the developing sac are here.
It works in wood only, with little machinery, and is best developed for children of from eleven to fifteen. It no longer aims to make artisans; but its manipulations are meant to be developmental, to teach both sexes not only to be useful but self-active and self-respecting, and to revere exactness as a form of truthfulness.
These relationships, as also this developmental power, will doubtless, in a certain sense, be somewhat further explained as science advances. But the result will be merely a shifting of the inexplicability a point backwards, by the intercalation of another step between the action of the internal condition or power and its external result.
How well warranted are we therefore in identifying the latter with the former. Equally scanty is our knowledge of the developmental history of the Ostracoda. The development of Cypris has recently been observed by Claus: "The youngest stages are shell-bearing Nauplius-forms." The section of the Branchiopoda includes two groups differing even in their development, the Phyllopoda and the Cladocera.
Finding thus that in the evolution of the social organism, as in the evolution of individual organisms, there are integrations as well as differentiations, and moreover that these integrations are of the same three orders; we have additional reason for considering these integrations as essential parts of the developmental process, needed to be included in its formula.
In exploring such sources we soon see how few writers have given true pictures of the chief traits of this developmental period, which can rarely be ascertained with accuracy. The adult finds it hard to recall the emotional and instinctive life of the teens which is banished without a trace, save as scattered hints may be gathered from diaries, chance experiences, or the recollections of others.
The arguments against such influence are perfectly sound: it is a primitive belief of unscientific origin; it is impossible to conceive how such influence can operate since there is no nervous connection between mother and child; comparatively few cases have been submitted to severe critical investigation; it is absurd to ascribe developmental defects to influences which arise long after the foetus had assumed its definite shape ; in any case the phenomenon must be rare, for William Hunter could not find a coincidence between maternal impressions and foetal marks through a period of several years, and Bischoff found no case in 11,000 deliveries.
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