38 MR. Ii. A. BORRADAILE ON THE 



only raised by the jaws of Malacostraca, but receives from the 

 study of some of them — notably from that of the maxilla — 

 considerable elucidation. The following, briefly stated, are the 

 considerations in regard to it which have influenced the 

 morphological suggestions put forward in the present paper. 



2. It is not to be assumed without question that a prototype 

 can be found from which all the limbs of all Crustacea can be 

 derived by modification. The mere fact that appendages stand 

 in the same position on the bodies of two or moi^e segmented 

 animals, or are membei-s of the same meristic series in one 

 animal, aflbrds no ground for the assumption that there exists 

 a common plan which underlies the arrangement of the parts of 

 each of them. Moreover, even when there are resemblances be- 

 tween them, such a conception as that of a common type of them 

 is of no use to the zoologist unless the plan of the type does 

 not mei-ely exist in the imagination of the observer, but has or at 

 some time had an objective existence as a factor in development. 

 Resemblances between two limbs may be due either solely to ex- . 

 trinsic causes — that is, to influences from without the organisms, 

 which have brought it about that different developmental pro- 

 cesses result in similar structures in the two cases, — or also to 

 intrinsic causes — that is, to the operation of identical develop- 

 mental factors. Only in the latter case has a common plan for 

 the two limbs an objective existence. Conceivably such a plan 

 may not be due to community of ancestry in a.ll instances. When 

 two appendages closely resemble one another — as do, for instance, 

 the various antenniform limbs of Arthropoda — it is possible, 

 and sometimes probable, that there exist in the mechanism of 

 development facilities for establishing such organs, and that 

 these have come into play independently more than once, forming 

 appendages upon the same plan. This principle, however, is of 

 limited application. Even in the cases which suggest it, it can 

 never be applied save to actual features which exist in all the 

 limbs under comparison. To assume the existence of a plan of 

 which some features are not realized in each limb is quite unjusti- 

 fied, except on the hypothesis of the common inheritance both of a 

 type of limb and, in the case of serial resemblance, of community 

 of type between the members of the series of limbs *. To look 

 for traces of a common type in structures in which it is so little 

 obvious as in the appendages Avhich appear to correspond in 

 diflferent Crustacea would be absurd, unless there were grounds 

 for believing tha,t their possessors were descended from a common 

 ancestor ; and if we are also to find community of type between 

 the several limbs of each individual crustacean, then such com- 

 munity must have existed among the appendages of the ancestor. 



* The resemblances here classed as due to extrinsic causes are those which are 

 generally classed under the head of Analogy. Those which are due to common 

 descent are instances of Honiologj'. Those which are due to independent operations 

 of the same developmental facilities (if such there be) form a third class not strictly 

 the same as either Analogy or Homology. 



