rent organs; or, in other words, that all the vital
movements are the product of impressions received by the sensitive
parts.[179]

"This cause seems, up to a certain point, established as regards the
most perfect animals; but had it been so relatively to all living
beings, they should all be endowed with the power of sensation. But
it cannot be proved that this is the case with plants, and it cannot
likewise be proved that it is so with all the animals known.

"But nature in creating her organisms has not begun by suddenly
establishing a faculty so eminent as that of sensation: she has had
the means of producing this faculty in the imperfect animals of the
first classes of the animal kingdom," referring to the Protozoa. But
she has accomplished this gradually and successively. "Nature has
progressively created the different special organs, also the
faculties which animals enjoy."

He remarks that though it is indispensable to classify living forms, yet
that our classifications are all artificial; that species, genera,
families, orders, and classes do not exist in nature--only the
individuals really exist. In the third chapter he gives the old
definition of species, that they are fixed and immutable, and then
speaks of the animal series, saying:

"I do not mean by this to say that the existing animals form a very
simple series, and especially evenly graduated; but I claim that
they form a branched series,[180] irregularly graduated, and which
has no discontinuity in its parts, or which, at least, has not
always had, if it is true that, owing to the extinction of some
species, there are some breaks. It follows that the _species_ which
terminates each branch of the general series is connected at least
on one side with other _species_ which intergrade with it" (p. 59).

He then points out the difficulty of determining what are species in
certain large genera, such as Papilio, Ichneumon, etc. How new species
arise is shown by observation.

"A number of facts teaches us that in proportion as the individuals
of one of our species are subjected to changes in situation,
climate, mode of life or habits, they thereby receive influences
which gradually change the consistence and the proportions of their
parts, their form, their faculties, even their structure; so that it
follows that all of them after a time participate in the changes to
which they have been subjecte