In the late 19th century, in a dispute between supporters of the adequacy of natural selection and those who held for inheritance of acquired characteristics, one of the arguments made repeatedly by Herbert Spencer, and followed by others, depended on what Spencer referred to as "co-adaptation" of "co-operative" parts, as in: "We come now to Professor Weismann's endeavour to disprove my second thesis — that it is impossible to explain by natural selection alone the co-adaptation of co-operative parts. It is thirty years since this was set forth in "The Principles of Biology." In §166, I instanced the enormous horns of the extinct Irish elk, and contended that in this and in kindred cases, where for the efficient use of some one enlarged part many other parts have to be simultaneously enlarged, it is out of the question to suppose that they can have all spontaneously varied in the required proportions." Darwin responded to Spencer's objections in chapter XXV of "The Variation of Animals and Plants Under Domestication" (1868). The history of this concept in the dispute has been characterized: "An older and more religious tradition of idealist thinkers were committed to the explanation of complex adaptive contrivances by intelligent design. ... Another line of thinkers, unified by the recurrent publications of Herbert Spencer, also saw co-adaptation as a composed, irreducible whole, but sought to explain it by the inheritance of acquired characteristics."