1916-17.] Experiments and Observations on Crustacea. 87 
setting we obtain a new outlook. Isopodan taxis is specialised, an 
arrangement associated with increased clinging power. Here we see 
a possibility of establishing new phylogenetic relations between the 
various groups The Epicaridea may well be an offshoot from the more 
typical Flabellifera ; the AAlvifera, the Flabellifera, and perhaps the 
Oniscoidea may represent divergent branches from a common clinging 
stock with fused coxal plates (on this conception the tanaidacean taxis 
of the Arcturidse would represent a secondary reversion, just as the 
tanaidacean (?) taxis in the Asellota is most probably a reversion). 
There is no need to pursue these speculations further ; they are not by 
any means proved. My object is rather to show how, when the purely 
morphological method slows down or comes to a standstill, it may be 
reinforced by the physiological or functional. In other parts of this 
paper we have had further opportunity of proving the value of this 
latter method (which is merely a reversion to that so elaborately used 
by Cuvier) applied to a task nowadays considered to be the exclusive 
province of morphology. 
Why physiology ceased to co-operate with morphology in this work 
is a question that belongs to the history of biology. Keith Lucas 
(1909), in an acute essay on functional evolution, has traced the origin 
of the anomaly to the period when a distinction first began to be 
drawn between homology and analogy. To his essay, perusal of which 
I can earnestly recommend to the reader, I should like to add a codicil. 
Analogy and Homology . — According to Lankester (1888) it was 
Owen * who “ gave precision and currency to the morphological doctrines 
which had taken their rise in the beginning of the century by the 
introduction of two terms ‘ homology ’ and ‘ analogy,’ which were defined 
so as to express two different kinds of agreement in animal structure, 
which, owing to want of such ‘ counters of thought,’ had been hitherto 
continually confused. Analogous structures in any two animals com- 
pared were by Owen defined as structures performing similar functions, 
but not necessarily derived from the modification of one and the same 
part in the £ plan ’ or ‘ architype,’ according to which the two animals 
compared were supposed to be constructed. Homologous structures 
were such as, though greatly differing in appearance and detail from 
one another, and though performing widely different functions, yet 
were capable of being shown by adequate study of a series of inter- 
mediate forms to be derived from one and the same part or organ of 
the ‘ plan-form ’ or ‘ architype.’ It is not easy to exaggerate the service 
* Lankester appears here to have been in error. See Huxley (1894). 
