6 
animal in which the direction of the life-stream is periodically 
reversed; and, to continue our simile , this new experience is com- 
parable to that of the traveller who has accidentally stumbled upon 
an unexplored district presenting unique geographical features. Here, 
then, you perceive, is a natural phenomenon utterly at variance with 
the investigator’s former experience; and this fact, whilst it cogently 
illustrates, on the one hand, the danger of drawing general conclu- 
sions from the study of isolated genera or data furnished within 
a very limited area, does not, on the other hand, destroy the value of 
a wider generalisation. The physico-vital laws regulating the flow 
of blood in Salpa are the same as those operating throughout the 
entire animal series ; but the degree , direction , and conditional cir- 
cumstances under which they act have given rise to phenomena not 
only unusual, but without parallel in the domain of the Physiolo- 
gist’s experience; and I have thus shown that in one department 
of Biology, at least, the Naturalist epitomises the practical know- 
ledge of the traveller. 
Again, I would ask you to bear in mind that the traveller or geo- 
graphical geologist deals with phenomena as they occur in space. 
An evening’s discourse would fail me to show you by illustra- 
tion that phenomena in space have their dissimilar counterparts in 
time. 
This idea, indeed, has been in some measure substantiated by the 
late Professor Edward Eorbes, in his discourse ‘ On the Manifesta- 
tion of Polarity in the Distribution of Organised Beings in Time,’ 
delivered in the theatre of this Institution on the evening of Eriday, 
28th April, 1854. Nevertheless, calling to our aid the inductions 
of the Paleontologist, the Experimental demonstrations of the Phy- 
sicist, and the sublimer researches of the Astronomer, we are led to 
assume, by a species of analogy perfectly legitimate, that the laws 
operating throughout space are the same as those in time ; and there- 
fore I have submitted to your judgment a broad hypothesis, which, 
whether roughly or gently handled, is valuable only in so far as 
it reflects the harmonious outlines of inestimable truth. 
Were it desirable, by a further reference to existing facts, to in- 
crease the force of our hypothesis, for this purpose any of the more 
remarkable chemico-physico-vital phenomena which ordinarily pre- 
sent themselves to the mind of the Biologist might with almost 
equal advantage be adduced, and the truth thereby be i^endered 
more conspicuous. Quitting, however, this special application of 
the subject, I have further to observe, that throughout the entire 
range of organised existences — whether animal or vegetable — there 
may be traced a community of plan both in structure and develop- 
ment ; and it is whilst engaged in the study of any department of 
Natural History knowledge, that we are sure to recognise its 
influence in the aspect of an all-pervading law. In every natural 
group of individualities, or of separate organs — such as those 
of special sense, for example,— the working of this law is more or 
