no purpose, and serving no end, exist in conformity to an ideal archetype on which the bony skeletons of all vertebrated animals are formed,[271] and which has never been departed from since time began. This type, or model, or plan, is, however, itself an evidence of _design_ as much as the plan of a house. For to what standard are we referring when we say that two limbs are morphologically the same? Is it not an _ideal_ plan, a _mental_ pattern, a metaphysical conception? Now an _ideal_ implies a mind which preconceived the idea, and in which alone it really exists. It is only as "an _order of Divine thought_" that the doctrine of animal homologies is at all intelligible; and Homology is, therefore, the science which traces the outward embodiment of a Divine Idea.[272] The principle of intentionality or final causation, then, is not in any sense invalidated by the discovery of "a unity of plan" sweeping through the entire universe. [Footnote 270: Carpenter's "Comparative Physiology," p. 37.] [Footnote 271: Agassiz, "Essay on Classification," p. 10.] [Footnote 272: Whewell's "History of Inductive Sciences," vol. i. p. 644; "The Reign of Law," p. 208; Agassiz, "Essay on Classification," pp. 9-11.] We conclude that we are justly entitled to regard "the principle of intentionality" as a primary and necessary law of thought, under which we can not avoid conceiving and describing the facts of the universe--_the special adaptation of means to ends necessarily implies mind_. Whenever and wherever we observe the adaptation of an organism to the fulfillment of a special end, we can not avoid conceiving of that _end_ as foreseen and premeditated, the _means_ as selected and adjusted with a view to that end, and creative energy put forth to secure the end--all which is the work of intelligence and will.[273] And we can not describe these facts of nature, so as to render that account intelligible to other minds, without using such terms as "contrivance," "purpose," "adaptation," "design." A striking illustration of this may be found in Darwin's volume "On the Fertilization of Orchids." We select from his volume with all the more pleasure because he is one of the writers who enjoins "caution in ascribing intentions to nature." In one sentence he says: "The _Labellum_ is developed into a long nectary, _in order_ to attract _Lepidoptera_; and we shall presently give reasons for suspecting the nectar is _purposely_ so lodged that it can be sucked only slowly, _in order_ to give time for the curious chemical quality of the viscid matter settling hard and dry" (p. 29). Of one particular structure he says: "This _contrivance_ of the guiding ridges may be compared to the little instrument sometimes used for guiding a thread into the eye of a needle." The notion that every organism has a use or purpose seems to have guided him in his discoveries. "The strange position of the _Labellum_, perched on the summit of the column, ought