28 BIOLOGICAL LECTURES. 



we are permitted to draw certain inferences as to the general 

 nature of the controlling forces. That such changes may, in 

 particular cases, be brought about quickly, so that the results 

 become more or less apparent within the space of an average 

 life, and thus form the basis of broader generalizations, is well 

 known ; but in the main they have progressed but slowly, and 

 enormous intervals of time have been required for the transi- 

 tion from one type to another. We are not permitted to con- 

 ceive that this line of descent takes the form of a strictly lineal 

 succession. On the other hand, there is strong evidence in 

 support of the view that our mental figure must be that of a 

 deliquescent tree in which the main line of descent, or stem, 

 is dissolved into a series of great limbs or primary divisions. 

 These again branch repeatedly, until in the final division all 

 direct connection with the original stem is lost. As certain of 

 these lesser branches retain a full measure of vigor and persist 

 in their development to the very end, so others attain their 

 highest development at a comparatively early stage, enter upon 

 a period of decline, and shortly disappear, thereby introducing 

 a further important disturbance of the natural arrangement 

 whereby it becomes increasingly difficult to determine the pre- 

 cise order of development and relationship between any two 

 members. And so with plants as a whole. Certain side lines 

 of descent have not yet attained their full development ; others 

 are now far along in their period of decline, which may have 

 had its origin in very remote geological times ; while yet others 

 have long since disappeared altogether, leaving no visible sign 

 of their former presence. And thus the symmetry of the bio- 

 logical tree is disturbed to such an extent and in such a way 

 that the remaining members appear to have little in common, 

 and sometimes even stand forth as isolated groups which seem 

 to justify the ancient idea of special creation. Fortunately the 

 situation is saved, in the first instance, by an appreciation of 

 the fact that, within certain limits, each individual reproduces 

 in the course of its life history the history of the group. 

 From this it follows that, however divergent species or groups 

 may appear in their fully matured state, the relationship may be 

 ascertained through their embryological phases and through 



