16 LIVERPOOL BIOLOGICAL SOCIETY. 
positive column, and that is why I say that the matter 
must be settled by an appeal to investigators, and by a 
search for fresh facts, and one of the very best fields for 
investigations of this nature lies amongst the Marine 
Invertebrata. 
Most writers upon evolution, following the example of 
Darwin and Wallace, have taken their examples from the 
higher animals—generally from the mammals and the 
birds, with the addition of insects. Consequently, most 
groups of the Invertebrata still present a virgin field for 
this department of evolutionary study.* I have commenced 
during the last couple of years some observations upon the 
nature of the diagnostic characters in the Nudibranchiate 
Mollusca and the Tunicata, and I shall now give you some 
examples from these groups in support of my views. 
NUDIBRANCHIATA. 
Let us take the Nudibranchiata first, and here one is at 
once met with by the difficulty of determining what the 
true specific characters are. If we turn to our great 
English work of reference, Alder and Hancock’s Mono- 
sraph of the Nudibranchiata, published by the Ray Society, 
we find that the shape, number and colour of the various 
projections from the body and other similar external 
features are employed as the diagnostic characters ; 
while in the works of Dr. Rudolph Bergh, the greatest 
living authority on Nudibranchs, and the author of the 
‘““Challenger’”” Report, the specific descriptions contain 
detailed accounts of the anatomy of all the important 
organs of the body, which are admirable contributions 
to knowledge, but do not single out for us those salient 
* Which is to a large extent that aspect of Zoology to which Ray Lankester 
has applied the term ‘‘ Bionomics.” Ency. Brit., 9th Ed., Art. Zoology. 
