440 PROFESSOR LANKESTER. 



particular case, is this — that the general doctrine of evolu- 

 tion justifies us in assuming, at some period or other, a 

 progression from the simplest to the most complicated 

 grades of structure ; that we are warranted in assuming at 

 least one progressive series leading from the monoplast to 

 man; and that until \oe have special reason to take a 

 different view of any particular case we are bound to 

 make the smallest amount of assumption by assigning 

 to the various groups of organisms the places which they 

 will fit into, on the supposition that they do represent in 

 reality the original progressive series. Nevertheless, any 

 naturalist would find it very difficult to prove, or render 

 highly probable, that many of the Protozoa are not descended 

 from Enterozoa by degeneration ; that Dicyema is not a de- 

 generate flat worm ; that the whole race of corals and polyps 

 are 7iot degenerate descendants of far more highly developed, 

 worm-like, free-swimming ancestors. 



When, therefore, the hypothesis of degeneration presents 

 itself as a solution of any special morphological difficulty, 

 we need have no scruples or prejudices in favour of the doc- 

 trine of universal progression, which should prevent us from 

 accepting it. 



In the following tables of classification I have made use 

 of the term ''phylum," proposed by Haeckel, instead of 

 "sub-kingdom," " embranchement/' or *' typus.^' The 

 phyla are so many great diverging branches of the family 

 tree of animals. Classes are the branches borne by phyla, 

 orders are the branches borne by classes. I have introduced 

 the term " branch" as an equivalent for subphylum, or sub- 

 class, as the case required. 



The most important explanation which is necessary here 

 is with regard to the terms " grade" and *' appendix,^' which 

 I have frequently introduced. Whilst all the other terms 

 indicate branches of the pedigree diverging from a very 

 nearly common point (thus all the classes in a phylum are 

 supposed to diverge from one common point unless it is 

 otherwise indicated), the various " grades" are introduced to 

 separate the starting-points of the branches; a certain 

 advance in differentiation of structure separates the branches 

 of a higher grade from those of a lower. An ' appendix ' is 

 an assemblage of ' degraded' examples of the group to which 

 it belongs. 



