TYPOLOGY 9 



applied to phylogeny, the origin of the leading 

 branches of the tree of life. 



Morphology in general treats of the evolution 

 of forms and organs within the compass of a 

 given type. Phylogeny does this too, and 

 more besides, since it is concerned with the 

 almost impossible task of tracing the pedigrees 

 of the primary types themselves. A little 

 explanation may be necessary at this point to 

 render my meaning intelligible. The word 

 "type" has been used in many different senses, 

 and is consequently a dangerous equivocal word 

 to trifle with, though very convenient. Examples 

 of types of the first order of magnitude are 

 the platyhelminth type, the annelid type, the 

 arthropod type, the molluscan type, the verte- 

 brate type. Probably in consequence of a 

 reaction against the old dogmatic use of the 

 term, these types have, at one time or another, 

 been connected together by imaginary links in 

 a manner which in many cases, is now recog- 

 nised as being quite inadmissible or, as we may 

 otherwise express it, in a manner contrary to 

 the canons of the art of morphology. Hopeless 

 pictures and impossible pedigrees have resulted 

 from this confusion of ideas. The criteria of 

 homology are indeed hard to define, being partly 

 intuitive, but we may find some indications which 

 may assist towards their future formulation. 

 The fish type, the bird type, and the mammalian 



