PREFACE. 



THIS book is offered as a contribution to the study of the 

 problem of Species. The reasons that have led to its production 

 are as follows. 



Some years ago it was my fortune to be engaged in an investi- 

 gation of the anatomy and development of Balanoglossus. At the 

 close of that investigation it became necessary to analyze the 

 meaning of the facts obtained, and especially to shew their bear- 

 ing upon those questions of relationship and descent which modern 

 morphology has attempted to answer. To this task I set myself 

 as I best might, using the common methods of morphological 

 argument and interpretation, and working all the facts into a 

 scheme which should be as consistent as I could make it. 



But the value of this and of all such schemes, by which each 

 form is duly ushered to its place, rests wholly on the hypothesis 

 that the methods of argument are sound. Over it all hung the 

 suspicion that they were not sound. This suspicion seemed at that 

 time so strong that in preface to what I had to say I felt obliged 

 to refer to it, and to state explicitly that the analysis was under- 

 taken in pursuance of the current methods of morphological 

 criticism, and without prejudging the question of possible or even 

 probable error in those methods. 



Any one who has had to do such work must have felt the same 

 thing. In these discussions we are continually stopped by such 

 phrases as, " if such and such a variation then took place and was 

 favourable/' or, " we may easily suppose circumstances in which 

 such and such a variation if it occurred might be beneficial," and 

 the like. The whole argument is based on such assumptions as 

 these assumptions which, were they found in the arguments of 

 Paley or of Butler, we could not too scornfully ridicule. " If," say 

 we with much circumlocution, " the course of Nature followed the 



