5 ORTHOGENESIS 79 



forms could not have been chemically identical with 

 that of the substance which went in the other direction, 

 and that, nevertheless, under the influence of light, the 

 same organ has been constructed in the one case as in 



o 



the other ? 



The more we reflect upon it, the more we shall 

 see that this production of the same effect by two 

 different accumulations of an enormous number of 

 small causes is contrary to the principles of mechan 

 istic philosophy. We have concentrated the full force 

 of our discussion upon an example drawn from phylo 

 genesis. But ontogenesis would have furnished us 

 with facts no less cogent. Every moment, right before 

 our eyes, nature arrives at identical results, in some 

 times neighbouring species, by entirely different em- 

 bryogenic processes. Observations of &quot; heteroblastia &quot; 

 have multiplied in late years, 1 and it has been necessary 

 to reject the almost classical theory of the specificity 

 of embryonic gills. Still keeping to our comparison 

 between the eye of vertebrates and that of molluscs, 

 we may point out that the retina of the vertebrate is 

 produced by an expansion in the rudimentary brain of 

 the young embryo. It is a regular nervous centre which 

 has moved toward the periphery. In the mollusc, on the 

 contrary, the retina is derived from the ectoderm directly, 

 and not indirectly by means of the embryonic encephalon. 

 Quite different, therefore, are the evolutionary processes 

 which lead, in man and in the Pecten, to the develop 

 ment of a like retina. But, without going so far as to 

 compare two organisms so distant from each other, we 



1 Salensky, &quot;Heteroblastie&quot; (Proc. of the Fourth International Congress of 

 Zoology, London, 1899, pp. m-ii8). Salensky has coined this word to 

 designate the cases in which organs that are equivalent, but of different 

 embryological origin, are formed at the same points in animals related to 

 each other. 



