INCISURA (sCISSUfiELLA) LYTTELTONENSIS. 43 



clianges in certain organs of developing embryos, it is no 

 longer possible to reject such suppositions as fanciful and 

 incredible. 



Those Avho have given unprejudiced consideration to the 

 objections raised against the all-sufficiency of natural selec- 

 tion, must have felt that a term is wanting somewhere in the 

 current forms of argument used to explain resemblances 

 between structures which are only doubtfully homogenetic. 

 The missing term may possibly be found when we have a 

 more exact knowledge of the kinds of factors whose co-opera- 

 tion is necessary to produce specific structure. Some of these 

 factors must be germinal, but evidence is accumulating that 

 germinal factors are not simple but compound, and may be 

 split into subordinate factors which, taken alone, do not pro- 

 duce the specific result. There is further evidence that 

 germinal factors react differently to different external factors, 

 and if this be so many kinds of resemblances and differences 

 may be accounted for by laws of interaction of which we are 

 as yet only dimly aware. 



The evidence on these matters is insufficient to enable us to 

 arrive at definite conclusions, but it is at any rate sufficient 

 to earn respect for a suggestion supported by such a large 

 number of positive facts as that of Osborn. 



I believe that in the future morphologists, in conjunction 

 with systematists, will be largely occupied in attempting to 

 discriminate between the different kinds of resemblances 

 among animal structures, between similarities due to the 

 " commoii action of evoking action or moulding environment,^' 

 and similarities due to direct descent, and I venture to think 

 that such morphological studies, carried out with sci'upulous 

 attention to detail, are not useless, but will give precision to, 

 and perhaps modify our views on, the causation of modifica- 

 tion of animal structure. 



