of the Animal Kingdom. 319 
jective and truly phylogenetic system of nature. For the 
present, however, we may be content with the result if we 
succeed in approaching this final aim as nearly as possible, 
and in course of time at least in constructing such a system 
as will represent in part, although not perfectly, the true 
genealogical tree of the animal kingdom. 
We will here at once look somewhat closely into those 
difficulties and obstacles which may not unfrequently confront 
the naturalist in his zealous endeavours to determine accu- 
rately the relationships of animals, and which readily give 
rise to errors. 
1. Sometimes the difficulty may consist in this, that in the 
case of organisms with accordant structures one may not un- 
frequently be inclined to conclude, from the similarity of 
structure, that they are true blood relations, and further that 
this affinity must be closer the greater their similarity of 
structure is found to be. The cautious naturalist, however, 
will soon perceive that this principle, if very generally applied, 
easily leads to errors, for in judging of a true blood-relationship 
that resemblance only can be of value which is produced origi- 
nally by inheritance, but not that in which the agreement in 
structure has merely resulted from adaptation to similar vital 
conditions. 
The judgment of these circumstances may sometimes appear 
very difficult, in many cases even impossible. This, then, is 
a difficulty which may easily lead the Zoologist astray by 
uniting in one and the same branch of the genealogical tree 
animals which are bound together by no close family tie, and 
this merely because they show an accordant structure, or 
because their larvee or embryos prove to be more or less alike. 
In phylogenetic classification therefore the zoologist must 
above all be able to distinguish well and accurately the homo- 
logy originating by inheritance, or the so-called homogeny, 
from a similarity produced by adaptation or homoplasy*. 
Observation shows that by the action of similar conditions 
of life, and therefore merely by adaptation, similar and some- 
times almost exactly identical organs may often be developed 
in animals which belong to perfectly distinct types or natural 
groups. As examples of this we may cite the chele occurring 
on the legs of crabs, and the palpi and chelicera of scorpions, 
as well as the pedicellariz of the Echinodermata, all of which 
show a wonderful agreement both in structure and function, 
although no one would venture to assert that they have actu- 
ally originated from each other by inheritance. In homo- 
* Ray Lankester, ‘On the use of the Term ‘ Homology,’ in Modern 
Zoology,” Ann. & Mag. Nat. Hist., 1870. 
