1S2 



SAMUEL H. SCUDDEE ON THE 



in Petrablattina, and in Archiraylaeris) are where only a couple of species or so are concerned. 

 The relative average size of the species of the different genera is also shown, and proves that 

 the average size of every genus of Mylacridse is larger than that of any of the other genera 

 excepting only Archimylacris, which I have elsewhere pointed out was the most antique 

 type of all cockroaches. The table further lends support to the view that the Dunkard 

 Creek series of rocks, in Monongalia Co., W. Va., are older than the Barren Coal-measures 

 and should be referred to the Permian ; since, in each of the three genera represented in 

 both beds, the average size of the species from the Dunkard Creek series is the smaller. 



AVERAGE LENGTH OF FORE WINGS IN THE GENERA OF AMERICAN PRE-TERTIARY COCKROACHES. 



In closing I wish to draw attention to a topic unusual in such a connection. In stndjiiig 

 protective resemblance and mimicry among living animals, the exceedingly common occur- 

 rence of these phenomena has often forced upon me the conclusion that they have not been 

 limited in their scope to recent times, but must have existed in past epochs and even, to some 

 extent at least, in very remote epochs. This is a natural conclusion from the universality of 

 their present occurrence. Hardly an animal exists that does not actually owe its existence 

 to some feature or features in its form or colouring. This statement will doubtless appear 

 strong to those who are unacquainted with or have not considered the facts. Let me re-en- 

 force it in the words of one of its latest exponents, M. Félix Plateau, the well known 

 professor in the University of Gand. " The thesis I wish to sustain in agreement with 

 naturalists of high merit," he says, " would demonstrate that the phenomena [of mimicry] 

 are general ; that is to say, that there are hardly any animals which, in at least some one of 

 the stages of their existence, do not have recourse to imitation ; that in our own countries, 



