Classification is a logical process which consists in putting together those things which are like and keeping asunder those which are unlike; and a morphological classification, of course, takes notes only of morphological likeness and unlikeness.  So long, therefore, as our morphological knowledge was almost wholly confined to anatomy, the characters of groups were solely anatomical; but as the phenomena of embryology were explored, the likeness and unlikeness of individual development had to be taken into account; and, at present, the study of ancestral evolution introduces a new element of likeness and unlikeness which is not only eminently deserving of recognition, but must ultimately predominate over all others.  A classification which shall represent the process of ancestral evolution is, in fact, the end which the labors of the philosophical taxonomist must keep in view.  But it is an end which cannot be attained until the progress of palaeontology has given us far more insight than we yet possess, into the historical facts of the case.  Much of the speculative ‘phylogeny,’ which abounds among my present contemporaries, reminds me very forcibly of the speculative morphology, unchecked by a knowledge of development, which was rife in my youth.  As hypothesis, suggesting inquiry in this or that direction, it is often extremely useful; but, when the product of such speculation is placed on a level with those generalisations of morphological truths which are represented by the definitions of natural groups, it tends to confuse fancy with fact and to create mere confusion.  We are in danger of drifting into a new ‘Natur-Philosophie’ worse than the old, because there is less excuse for it.  Boyle did great service to science by his ‘Sceptical Chemist,’ and I am inclined to think that, at the present day, a ’Sceptical Biologist’ might exert an equally beneficent influence.

[Sidenote:  Physiology.]

Whoso wishes to gain a clear conception of the progress of physiology, since 1837, will do well to compare Mueller’s ‘Physiology,’ which appeared in 1835, and Drapiez’s edition of Richard’s ’Nouveaux Elements de Botanique,’ published in 1837, with any of the present handbooks of animals and vegetable physiology.  Mueller’s work was a masterpiece, unsurpassed since the time of Haller, and Richard’s book enjoyed a great reputation at the time; but their successors transport one into a new world.  That which characterises the new physiology is that it is permeated by, and indeed based upon, conceptions which, though not wholly absent, are but dawning on the minds of the older writers.