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With this fifth line of thought a sixth is associated and intertwined. The problem of development is closely bound up with that of “heredity.” A developing organism follows the parental type. The acorn in its growth follows the type of the parent oak, repeating all its morphological and physiological characters down to the most intimate detail.
That part of biological science which deals with form and structure is called Morphology that which concerns itself with function, Physiology so that we may conveniently speak of these two senses, or aspects, of "species" the one as morphological, the other as physiological.
His unit of value is a morphological cell, a derivative organism rather than a primary vital unit; and all organisms are, as we have before said, destructible by heat. Professor Agassiz is pretty good authority for doubting the existence of such a cell.
But while it may safely be admitted that we are acquainted with only the morphological characters of the vast majority of species the functional or physiological, peculiarities of a few have been carefully investigated, and the result of that study forms a large and most interesting portion of the physiology of reproduction.
It would be absurd to claim that our present English pattern is identical with the old Indo-European one, yet it is impressive to note that even at this late day the English series of initial consonants: The relation between phonetic pattern and individual sound is roughly parallel to that which obtains between the morphologic type of a language and one of its specific morphological features.
Morphological or comparative studies need a material standard, by which it may be readily decided whether certain groups of animals and plants are to be described or de-nominated as species, as subspecies or as varieties. To get at the inner nature of the differences is in most cases impossible, but a decision must be made.
This supposition is not adverse to the derivation of species from a common stock, but rather favours the notion that as in the case of the individual the period of plasticity is short compared with that of morphological stability, so if there was such an arboreal branching out of species from a common root, it took place rapidly in conditions as different from ours as those of uterine from extra-uterine life; and that the stage of inflexibility may have been reached before any time of which we have record.
Upon a derivative theory this morphological conformity is explained by community of descent; and it has not been explained in any other way.
If we consider, in fact, that by far the largest proportion of recorded existing species are known only by the study of their skins, or bones, or other lifeless exuviae; that we are acquainted with none, or next to none, of their physiological peculiarities, beyond those which can be deduced from their structure, or are open to cursory observation; and that we cannot hope to learn more of any of those extinct forms of life which now constitute no inconsiderable proportion of the known Flora and Fauna of the world: it is obvious that the definitions of these species can be only of a purely structural, or morphological, character.
I believe that such influences may be demonstrated and that they deserve far more careful study than they have received. Was the pre-Anglo-Saxon alternation of fot and föti an absolutely mechanical matter, without other than incidental morphological interest? It is always so represented, and, indeed, all the external facts support such a view.
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