GEOLOGICAL SUCCESSION OF THE ECHINI. 7;,;; 



It is indeed difficult to imagine, from the little we know regarding the 

 habits of Echini, what are the ways in which natural selection is to act, not 

 only upon them, but upon many of the lower marine animals of which the 

 successive stages of life are so varied, and in which life is passed under such 

 very varying circumstances. All that a careful study and comparison of the 

 Echini, both living and fossil, enables us to assert is, that there is a marked 

 coincidence between the geological succession of the generic types and the 

 genetic succession observed during the changes due to growth ; that in the 

 growth and development of the species sudden breaks appear, similar to the 

 gaps observed in the geological succession of the types of the best known 

 strata. From this we are warranted in assuming that the mode of appear- 

 ance of generic types in past ages is analogous to that observed in the his- 

 tory of the species. But how this appeai'ance takes place we are as far from 

 knowing as we are of giving the reason for the successive stages in the seg- 

 mentation of the embryo. 



It may not be out of place here to recall that Agassiz was the first to point 

 out in his " Poissons Fossiles " the agreement existing between the paleonto- 

 logical and the embryological genesis, though he did not draw from it the 

 conclusions of a common origin subsequently suggested by Darwin. It is 

 astonishing that so little use has been made of the positive data furnished by 

 embryology in support of the evolution hypothesis, and that so many of the 

 supporters of the Darwinian theory have been satisfied to build castles in 

 the air, which they have been obliged to pull down in rapid succession, till 

 it is wellnigh impossible, in the present stage of the discussion of the embryo- 

 logical evidence, to sift from the best memoirs what has actually been ob- 

 served, and what has no better foundation than a mere theoretical basis. To 

 those who have followed the discussions of the embryology of articulates the 

 above will not appear too strong a statement. It is only in a few orders of 

 the animal kingdom that we have even the first beginnings of the needed 

 paleontological and embryological material to serve as the basis for the com- 

 parisons which might lead to some definite results. Yet these comparisons 

 are generally instituted on such a grand scale, and with such an utter dis- 

 regard of the exceptions, that their authors can hardly expect us to follow 

 them in the paths they tread, where theory takes the place of observation. 

 No one appreciates more than I do that the explanation of the theory of 

 evolution, as given by Darwin, has opened up new fields of observation in 

 many departments of biology, the importance of which can hardly be over- 



