THE THEORIA GENERATIONIS. 267 



manner (aet ov, Kai firjSeTroTe ovre f^iyvo/jievov, ovre cnroWv/xevov). 

 If we can recognize the zdea in this animal, it is immaterial 

 and unimportant whether we are looking at the animal now 

 before us or its ancestor that liyed a thousand years ago, or 

 whether the animal is here or in a distant land, or whether it 

 appears in this or that manner, position or action, whether, 

 finally, it be this or another individual of the same species : all 

 this is unessential and appertains only to appearances : the idea 

 of the animal alone really is, and really is an object of the 

 understanding." ^ 



It would not be difficult to trace the Heraclitean conception 

 of the flux through Aristotle down to such modern philosophers 

 as Hegel and Herbert Spencer, and to trace the Platonic idea, 

 through the X0709 cnrepfxaTLKO^ of the Stoics, \k\Q fonna S2ibstan- 

 tialis and the causae primordiales of the scholastics, to Kant's 

 Ding-an-sich, Schelling's Absolute, and the Platonic idea as 

 adopted by Schopenhauer. But the tracing of these concep- 

 tions in detail would lead us far afield in metaphysics. I should 

 beg your indulgence for mentioning these matters did they not 

 seem to me to be, in some measure, necessary to a proper 

 understanding of the two great views of embryonic development 

 that have been and still are held by thinking students of nature 

 — preformation and epigenesis. 



The development of the living organism is the most striking 

 special case of development we know. The development of 

 what appears to be a simple e.gg, within a comparatively short 

 time, and beneath our very eyes, into a complex living animal, is 

 development par excellence — the very perfection of that devel- 

 opment which is more dimly apprehended in the much slower 

 growth of worlds, of human societies and human institutions. 

 Hence we do not wonder that the development of the individ- 

 ual organism has become one of the main tests of two alterna- 

 tive views which, with a more general application, have from 

 the earliest times vexed philosophic thinkers. 



Under the influence of the Christian church the Platonic 

 conception seems to have led to the notion of the special crea- 



1 Schopenhauer, A. Die Welt als Wille und Vorstellung. Leipzig, Brock- 

 haus, 1888. Bd. i, p. 203. 



