afterwards, by the interaction of adaptation and heredity, became modified in form and differentiated in a vast variety of ways in the struggle for existence60.ã What Haeckel precisely signified by these words is not clear to me.
F. E. Schultze, perceiving that identical forms of spicule were developed whether the material were crystalline or non-crystalline, abandoned all theories based upon cryôÙstalôÙliôÙsaôÙtion; he simply saw in the form and arrangement of the spicules something which was ãbest fittedã for its purpose, that is to say for the support and strengthening of the porous walls of the sponge, and found clear evidence of ãutilityã in the specific structure of these skeletal elements. {455}
Sollas and Dreyer, as we have seen, introduced in various ways the conception of physical causation,ãas indeed Haeckel himself had done in regard to one particular, when he supposed the position of the spicules to be due to the constant passage of the water-currents. Though even here, by the way, if I understand Haeckel aright, he was thinking not merely of a direct or immediate physical causation, but of one manifesting itself through the agency of natural selection61. Sollas laid stress upon the ãpath of least resistanceã as determining the direction of growth; while Dreyer dealt in greater detail with the various tensions and pressures to which the growing spicule was exposed, amid the alveolar or vesicular structure which was represented alike by the chambers of the sponge, by the reticulum of constituent cells, or by the minute structure of the intracellular protoplasm. But neither of these writers, so far as I can discover, was inclined to doubt for a moment the received canon of biology, which sees in such structures as these the charôÙacôÙterisôÙtics of true organic species, and the indications of an hereditary affinity by which blood-relationship and the succession of evolutionary descent throughout geologic time can be ultimately deduced.
Lastly, Minchin, in a well-known paper62, took sides with Schultze, and gave reasons for dissenting from such mechanical theories as those of Sollas and of Dreyer. For example, after pointing out that all protoplasm contains a number of ãgranulesã or microsomes, contained in the alveolar framework and lodged at the nodes of the reticulum, he argued that these also ought to acquire a form such as the spicules possess, if it were the case that these latter owed their form to their very similar or identical position. ãIf vesicular tension cannot in any other instance cause the granules at the nodes to assume a