96 REPORT — 1873. 



agreement with a common plan — that they may be derived by inheritance from a 

 common ancestral form, and that they therefore afford evidence of a true blood- 

 relationsliip between the organisms presenting them. 



The recognition of this relationship is the basis of what is known as the Descent 

 Theory. No one doubts that the resemblances we notice among the members of 

 such small groups as those we name species are derived by inheritance from a 

 common ancestor ; and the Descent Theory is simply the extension to the larger 

 groups of this same idea of relationship. 



If this be a true principle, then biological classification becomes an exposition 

 of family relationship — a genealogical tree in which the stem and branches indi- 

 cate various degrees of kinship and direct and collateral lines of descent. It 

 is this conception which takes classification out of the domain of the purely 

 morphological. 



Affinity determined hy the study of Anatomy and Devidopment. 



Prom what has just been said, it follows that it is mainly by a comparison of 

 organisms in their anatomical and developmental characters that their affinities 

 are discoverable. The structure of an organism will, in by far the greater number 

 of cases, be sufficient to indicate its true affinity ; but it sometimes happens that 

 certain members of a group depart in their structure so widely from the characters 

 of the t3'pe to which they belong, that without some other evidence of their affi- 

 nities no one would think of assigning them to it. This evidence is afibrded by 

 development. 



An example or two will serve to make the subject clear; and we shall first take 

 one from a case where, without a knowledge of anatomical structure, we should 

 easily go astr.ay in our attempts to assign to the forms under examination their 

 true place in the classification. 



If we search our coasts at low water we shall be sure to meet with certain 

 plant-like animals spreading over the rocks or rooted to the fronds of sea-weeds, 

 all of which present so close a resemblance to one another as to have led to their 

 being brought together by the zoologists of a few years ago into a single group, 

 to which, under the name of " Polypes," a definite place was assigned in the 

 classification of the animal kingdom. They are all composite animals, consisting 

 of an association of buds or zooids which remain organically imited to one another 

 and give to the whole assemblage the appearance, in many cases, of a little 

 branching tree. Every bud carries a delicate transparent cup, within which is 

 contained the principal part of the animal, and from which this has the power of 

 spontaneously protruding itself ; and when thus protruded it will be seen to pre- 

 sent a beautiful crown of tentacles surrounding a mouth, througli which food is 

 taken into a stomach. As long as no danger threatens, the little animal will 

 continue displayed with its beautiful coronal of tentacles expanded ; but touch it 

 ever so lightly, and it will instantly close up its tentacles, retract its whole body, 

 and take refuge in the recesses of its protecting cup. 



So far, then, there is a complete agreement between the animals which have 

 been thus associated under the designation of Polypes ; and in all that concerns 

 their external form no one point can be adduced in opposition to the justice of 

 this association. When, however, we pass below the surface and bring the micro- 

 scope and dissecting-needle to bear on their internal organization, we find that 

 among the animals thus formed so apparently alike we have two totally distinct 

 types of structure : — that while in one the mouth leads into a simple excavation of 

 the body on which devolves the whole of the functions which represent digestion, 

 in the other there is a complete alimentary tract entirely shut oft' from the proper 

 cavity of the body and consisting of distinctly difterentiated oesophagus, stomach, 

 and intestine ; while in the one the muscular system consists of an indistinct 

 layer of fibres intimately imited in its whole extent with the body-walls, in the 

 other there are distinctly differentiated free bundles of muscles for the purpose of 

 effecting special motions in the economy of the animal ; while in the one no dif- 

 ferentiatect nervous system can be detected, in the other there is a distinct nervous 

 ganglion with nervous filaments. In fact the two forms are shown, by a study of 

 their anatomical structui-e, to belong to two entirely different primary divisions 



