144 THE EVOLUTION OF LIVING BEINGS. 



them different, and that we can do this just as well 

 with members of very different classes as with members 

 of the same class, by trimming their differences a little 

 more yet. 



It is easy, in this way, to construct a general ground- 

 plan even between a sea-urchin and a hedgehog, by 

 abstraction of all the differences they show, until we 

 have nothing left than the one point they have in 

 common: the covering with spines and then claim 

 that the groundplan of Echinus and Erinaceus al- 

 lows us, to unite them to the group of the Spiniferae, 

 which is not very much worse than the group of the 

 Chordata as defined in the Handworterbuch of the 

 Naturwissenschaften II p. 623 as : ,,die durch den Be- 

 sitz einer Riickensaite, Chorda dorsalis, ausgezeichne- 

 ten Tiere: Tunica ten, Amphioxus und Vertebraten." 



Man's mind is a curious one, because, after having 

 thus divested the different individuals he sees in na- 

 ture, of all their differences, he assigns such immense 

 importance to the shabby rest thus obtained, that 

 he proceeds to explain the differences (he has just 

 succeeded in explaining away) by speculations about 

 the causes which could have changed this common 

 rest to all the different structures which are actually 

 presented by the different organisms, forgetting enti- 

 rely, that there is not the slightest reason to consider this 

 rest as ever having been transformed. 



It is always the same mistake : looking for the (non 

 existing) single origin of different types. 



So one reaches all kinds of attractive, but quite un- 

 founded, conclusions as f . i. that the flapper of a seal 



