Notes on embryology and classification. 437 



the assumptions (1) that organisms of like structure — that 

 is, with like adaptations — are related to one another by 

 blood with a degree of closeness which is in direct proportion 

 to the closeness of the likeness ; (2) that the general effect of 

 evolution in relation to organisms has been to eifect a pro- 

 gress from simpler structural conditions to more complicated, 

 whence it is inferred that the more simple organisms which 

 to-day exist are surviving representatives of the earlier phases 

 of organic evolution, the race to which they belong never 

 having attained a higher level than it does to-day, and that 

 all existing organisms may be arranged according to the 

 degrees of complication of their structure in several ascend- 

 ing series, the degrees in which represent so many stages 

 attained to and passed through by the ancestors of the most 

 highly complicated member of the series. 



Having started with these two assumptions, as all those 

 who have attempted phylogenetic classification have done, it 

 is very soon found to be necessary to qualify and relax the 

 general application of both the principles assumed. It is 

 very soon recognised (and, indeed, is universally admitted) 

 that there are many cases of a pair of organisms which are, 

 on the whole or as to some striking detail, alike in structure 

 — that is to say similarly adapted — and yet (as we learn 

 from their developmental history or from some one indis- 

 putable structural feature) do not owe that similarity to 

 heredity, but to an independent identity in adaptation 

 occurring in the two cases in consequence of a recurrence of 

 the same adaptational conditions. Such similarity is said to 

 be due to homoplasy,i whilst hereditary likeness is due to 

 homogeny. In phylogenetic classification, then, we have to 

 be especially on our guard against mistaking homoplastic for 

 homogenetic agreements. 



An " objective" method of classification which should 

 ignore the doctrine of evolution could not fail to confuse 

 organisms related by homogeny with others related to them 

 only by homoplasy ; and, indeed, this was notoriously the case 

 with the classifications of the first half of this century. 



Again, as to the second assumption of "a continuous 

 progression" in all the myriad branches and twigs of 

 the organic family tree — the assumption of a continuous 

 flow onwards (slower or faster, but always forward) in all 

 the multifarious streamlets into which the original stream of 

 life has s ubdivided, this, too, has been universally qualified. 

 It has been admitted in certain very obvious cases, e. g. many 



^ This term was first proposed in my article " On the Use of the term 

 Homology in Modern Zoology," ' Annals and Mag. Nat. Hist.,' 1870, 



