CUSP RECTN; i!. \DATIO\S 239 



such a general character as the 'likeness of material' (noi of elaborated 

 form and parts) in two epidermal surfaces. Thus the beak nt' tin 1 

 liinl and of the turtle might be developed homoplastically from a 

 common ancestor's snout which had no beak but in both the bird- 

 line and the turtle-line horn-producing epidermis is tin- ehief material 

 bi-ouglit into greater development. 



"So in the bi-chambered hearts of bird and mammal the -unit, rial 

 is muscular tissue and the peculiar muscular tissue of the heart. 



" I should like to know what cases of ' convergence ' you can cite 

 in which there is not some remote homogeny as to the tissues at 

 work, which is a totally different thing from inherited community of 

 the special form considered. You would not call a four-chambered 

 seed-vessel and a four-chambered heart cases of convergence nor of 

 homoplasy ! (except in the very remotest degree which if entertained 

 makes all shape and structure in a minimal degree homoplastic with 

 all other). Can you name cases of convergence or parallelism which 

 are not covered by the definition I gave of homoplasy ? What organs 

 are parallel in any two animals and yet have no likeness at all even 

 the most general in their material. 



' Yours sincerely, 



" E. HAY LANKESTKK. 



" P.S. Every living thing owes its properties to homogeny, that 

 is to say, its fundamental properties to a homogeny common to it 

 with all other living things, and as you run down group within 

 group there must be a more and more specialized homogeny affecting 

 members of smaller and smaller groups. 



" But this factor can be detached in our consideration of structure 

 from the homogeny of actual completed form and mechanism. The 

 one is much more remote and less specific than the other ; and they 

 need to be given each its due rank and place ; not to be confused." 



o. CONCLUSION (OSBORN), 1900. 



The newly and independently arising cusps, described above as 

 ' homoplastic,' should be described as rcctigradations* 



*The term ' rectigradation ' was defined in Science, N.S., Vol. XXI., June 23, 1905, 

 p. 961, as follows: "Fourth, rectigradation, a new term with which I propose to charac- 

 terize what in the year 1889 I described as ' definite variations ' ; it embraces changes which 

 many writers have described as ' orthogenetic,' under the supposed law of direct change, 

 usually in an adaptive direction, which is described as Orthogenesis ; these probably are 

 the 'mutations' of Waagen.'' 



