304: PROCEEDINGS OF SOCIETIES. 
in character to Cythere, does not undergo these changes, and 
might furnish some ground for the supposition that these marine 
Entomostraca also do not; but it would be sufficient for the pre- 
sent purpose to state that there is no support for the supposition 
that they do, on the general ground that some kinds of Entomos- 
traca are known to undergo these changes, because it is also well 
known that other kinds do not. 
But a further objection was that low forms of animal life are 
liable to extreme variability, and that the same remark is applic- 
able to the lower groups in the higher divisions of animals, the 
argument being that in these cases the variability may be so great 
as to render specific distinction of little or no value. He felt 
compelled to state that he did not admit this excessive variability 
as a certain fact, even in the lowest forms of animal life, until it 
could be actually proved to be true by most careful observation, 
and even then he would apply it no further than to the particular 
eases in which it had been proved; but as to making it a general 
rule with the lowest groups of the higher divisions of animals, he 
saw not the slightest reason for doing so, and here, more than in 
the former case, the diversity of character in the different groups 
would render it extremely unlikely that what might apply to one 
would also be found to apply to another. Even among low forms 
of animal life, such as the Foraminifera, he had searched in vain 
for examples of that extreme variability which must necessarily 
confound specific distinctions, and he was sure that every young 
working naturalist, at all events, would agree with him that a con- 
clusion so discouraging to exact observations as that suggested by 
Professor Williamson, ought to be accepted by no one unless he be- 
came convinced of its truth by his own painful experience. With 
regard to the Foraminifera, he had no hesitation in saying that, 
with only a few exceptions, the extraordinary uniformity of cha- 
racter in individuals of the same type was a fact that must strike 
every observer. In the cases of Miliolina and Polymorphina he 
would admit there is difficulty, but an examination of many of 
their numerous forms did not suggest to his mind that a great 
tendency to variation, in the ordinary sense of the term, would 
explain their diversity. Knowing, however, from his own obser- 
vations, what is meant by the statement that the Foraminifera are 
liable to great variation, he was prepared to say that the shells of 
the marine Entomostraca do not at all show this liability; the 
forms are clearly defined, and distinct from one another, and in- 
termediate forms blending the characters of two others rarely, if 
eyer, occur. 
Then, as to the last objection which was brought forward, 
namely, the difficulty of defining what a species is, he believed 
that practically it might be put aside altogether ; for the fact was 
that any creature whatever which could be shown to be clearly 
distinct from all others that had been described must be admitted 
as a species, and must remain a species until it could be proved to 
be unworthy of this distinction. In conclusion, he would say that 
