100 REPORT— 1873. 



be regarded as representing them. We have only to imagine these stipules so 

 modified in their form as to become reduced to two long spiral threads, and we shall 

 at once have the tendrils of the Simla.v. On the other hand, let the stipules in our 

 type remain as leaf-like organs, and let the rest of the leaf (the lamina and petiole) 

 lose its normal character and become changed into a spiral thread, and we shall 

 then have the stipules of our type leaf retained in the two opposite leaf-like organs 

 of the Lathyrus, while the remainder of the type leaf will present itself in the con- 

 dition of the irt^/«/r«s-tendril which springs from the central point between them. 



The tendrils of the Smilax and of the Lathyrus aphaca are thus not homologous 

 with one another, but only analogous ; while those of the Smilax are homologous 

 with a pair of stipules, and those of the Lathyrus homologous with the lamina and 

 petiole of a leaf. 



Besides the homology discoverable between the organs of different animals and 

 plants, a similar i-elation can be traced between organs in tlie same animal or 

 plant, as, for example, tliat between the different segments of the vertebral column 

 (which can be shown to repeat one another homologically), and that between the 

 parts composing the various verticils of the flower and leaves in the plant. 



The existence of homological relations such as have been just illustrated admits 

 of an easy explanation by the application of the doctrine of Descent, according to 

 which the two organs compared would originate from a common ancestral form. 

 In accordance with this hypothesis, Homology would mean an identity of genesis 

 in two organs, as Analogy would mean an identity of function. 



Distribution and Evolution. 



Another very iniportant department of biological science is that of the distribu- 

 tion of organized beings. This may be either Distribution in Space (Geographical 

 Distribution) or Distribution in Time (Palfeoutological Distribution). Both of these 

 have of late years ac((uired increased significance ; for we have begun to get more 

 distinct glimpses of the laws by which they are controlled, of the origin of Faunas 

 and Floras, and of the causes which regulate the sequence of life upon the earth. 

 Time, however, wnll not allow to enter upon this subject as fully as its interest and 

 importance would deserve ; and a few words on palseontological distribution is all 

 that I can now venture on. 



The distribution of organized beings in time has lately come before us in a new 

 light, by the application to it of the hypothesis of Evolution. According to this 

 hypothesis, the higher groups of organized beings now existing on the earth's sur- 

 face have come down to us, with gi-adually increasing complexity of structure, by a 

 continuous descent from forms of extreme simplicity which constituted the earliest 

 life of our planet. 



In almost every group of the animal kingdom the members which compose it 

 admit of being arranged in a continuous series, passing down from more specialized 

 or higher to more generalized or lower forms ; and if we have any record of ex- 

 tinct members of the group, the series may be carried on through these. Now, 

 while the Descent hj'pothesis obliges us to regard the various terms of the series 

 as descended from one another, the most generalized fonns will be found among 

 the extinct ones ; and the further back in time we go the simpler do the forms 

 become. 



By a comparison of the forms so arranged we obtain, as it were, the law of the 

 series, and can thus form a conception of the missing terms, and continue the 

 series backwards through time, even where no record of the lost forms can be 

 found, untU from simpler to still simpler terms we at last arrive at the conception 

 of a term so generalized that we may regard it as the primordial stock, the ances- 

 tral form from which all the others have been derived by descent. 



This root form is thus not actually observed, but is rather obtained by a process 

 of deduction, and is therefore hypothetical. We shall strengthen, however, its 

 claims to acceptance by the application of another principle. The study of Em- 

 bryology shows that the higher animals, in the course of their development, pass 

 through transitory phases which have much in common with the permanent con- 

 dition of lower members of the type to which they belong, and therefore with ite 



