No. 3.] FACTORS IN THE EVOLUTION OF MAMMALIA. 391 



merely by their passive presence, and therefore could not be af- 

 fected by disuse, which we know very well is not the case. Prob- 

 ably no one imagines that the armor of crustaceans and insects 

 owes its origin to " Stiche und Bisse," or that its welfare in any 

 way depends upon them, but muscular activity, contact with the 

 water, light, etc., are essential. On the theory of panmixia 

 what is the probability that the reduction should reach exactly 

 as far as the tube or shell in which the animal lives ? It is a 

 matter of no consequence whether the armor extends a little 

 way into the tube or not ; why, then, such superfluous accuracy ? 

 Admitting that this might happen in the case of one species, it 

 becomes more and more improbable with every case observed. 

 On the other hand, if the presence of the tube or shell does 

 have a direct effect, this exact correspondence is just what 

 might be expected. Indeed, Weismann's explanation is contra- 

 dicted by a dictum which he lays down in another part of the 

 same essay, though it should be added that this proposition 

 refers primarily to positive differentiation. " Die Organismen- 

 welt beweist uns, dass dem so ist, denn bis ins Einzelnste hinein, 

 sehen wir jede lebende Art sich zweckmassig gestalten, als es 

 unumganglich nothwendig ist, urn sie existenzfahig zu machen, 

 nicJit urn ein Minimum mehr" (p. 30). 



The case of the neuter insects does, however, offer a for- 

 midable difficulty to the doctrine of the inherited effects of disuse, 

 as, indeed, it does to any explanation. It would lead too far 

 from the main subject of this paper to give to this problem the 

 full and extended discussion which it requires. I can, therefore, 

 only refer the reader to Eimer's examination of this question 

 (No. 15, p. 286 ff.), which it seems to me is quite as probable as 

 Weismann's, to say the least ; especially if one will carefully 

 follow out the steps of the probable process by which, for ex- 

 ample, the worker ants have lost their wings. But even if we 

 admit Weismann's position in regard to the neuter insects, it 

 would merely be saying that disuse is not the only factor in the 

 suppression of organs, an assumption which few would be bold 

 enough to make. At all events, Darwin, who first insisted upon 

 the difficulty presented by these insects, did not regard it as mil- 

 itating in any way against the assumption of the effects of dis- 

 use in other cases. 



The positive evidence in favor of the theory of panmixia is 



