DEVELOPMENTAL MECHANICS. I 77 



through the classes of each type, comparison of different forms 

 showed that essentially and unequivocally the same course of 

 progressive development is followed by nearly all systems of 

 organs. But in further approximations of a higher degree, 

 viz., in tracing that development through the orders, fami- 

 lies, genera, and species, even to the individual, so many in- 

 congruities in the development of organ systems and organs 

 made their appearance, that comparative anatomy has been 

 compelled to call in the assistance of quite a number of de- 

 velopmental mechanical hypotheses, for the correctness of 

 which only experimental tests can give complete security. 



Even the appt'eciation of "■essential or '^unessential agree- 

 ments or differences, an appreciation which is continually neces- 

 sary in the phylogenetic explanation of comparative observations 

 on form, in ultimate analysis always shows itself to be of a de- 

 velopmental mechanical character. 



Since developmental mechanics, perhaps for some time to 

 come, or at least in the beginning, will pursue its own course, 

 it would be encouraging if comparative anatomists would them- 

 selves resort to experimentation for the purpose of solving, so 

 far as possible in a short time, the problems in which they are 

 interested, e.g., the continually recurring main question, as to 

 what are actually — not in a formal, but in a developmental 

 sense — ''slight'' or "easy" variations; whether the number 

 of organs may be increased "easily" {i.e., by a simple inter- 

 ference and hence by a correspondingly slight accident), as 

 perhaps by the passive infolding of a somite, by the splitting 

 of a shoot, or by linear pressure on the same in a direction 

 contrary to its direction of growth, and further, in case these 

 attempts are successful, whether or not such newly formed 

 organs at once attain to the full differentiation of the former 



o 



ones; further, whether, inversely, a decrease in the number of 

 organs may be "easily" brought about, perhaps by inhibiting 

 a normal infolding or constriction or by compression and re- 

 sulting concrescence ; whether in these cases according to the 

 earlier or later stage of development, during which such inter- 

 ference is applied, the united parts may at once become per- 

 fectly simple or still retain traces of their double origin, etc. 



