ts of agreement or difference seemed greater or less when
set down as it were on a chart or map. They regard the small well-marked
series which have been styled natural families, as groups which should
be placed between the isolated species and their nearest neighbours so
as to form a kind of reticulation. This idea, which some of our modern
naturalists have held to be admirable, is evidently mistaken, and will
be discarded on a profounder and more extended knowledge of
organization, and more especially when the distinction has been duly
drawn between what is due to the action of special conditions and to
general advance of organization."[254]

I take it that Lamarck is here attempting to express what Mr. Charles
Darwin has rendered much more clearly in the following excellent
passage:--

"It should always be borne in mind what sort of intermediate forms must,
on the theory [what theory?], have formerly existed. I have found it
difficult when looking at any two species to avoid picturing to myself
forms _directly_ intermediate between them. But this is a wholly false
view; we should always look for forms intermediate between each species
and a common but unknown progenitor; and the progenitor will generally
have differed in some respects from all its modified descendants. To
give a simple illustration: the fantail and pouter pigeons are both
descended from the rock pigeon. If we possessed all the intermediate
varieties which have ever existed, we should have an extremely close
series, between both and the rock pigeon; but we should have no
varieties directly intermediate between the fantail and the pouter;
none, for instance, combining a tail somewhat expanded with a crop
somewhat enlarged, the characteristic features of these two breeds.
These two breeds, moreover, have become so much modified that, if we had
no historical or indirect evidence regarding their origin, it would not
have been possible to have determined, from a mere comparison of their
structure with that of the rock pigeon C. livia, whether they had
descended from this species, or from some other allied form, as C.
oenas.

"So with natural species, if we look to forms very distinct--for
instance, to the horse and the tapir--we have no reason to suppose that
links directly intermediate between them ever existed, but between each
and an unknown common parent. The common parent will have had in its
whole organization much general resemblance to the tapir