POEITES. 15 



up of at least two units, a calicle-unit and a colonial-unit, supplies us with a further instru- 

 ment for that purpose (see below § IV. p. 20). 



Hitherto all the forms included in the Division 1 (a, b, e) have been indiscriminately 

 put togetlier as belonging to the same species, Pwites astrceoides. It is (juite impossible 

 to deny that they may all be local variations of one and the same species, but thatis an 

 hypothesis which certainly does not advance knowledge one iota; on the contrary, it 

 arbitrarily closes the book. Duchassaing and Michelotti thought that the encrusting and 

 massive forms which they collected belonged not only to different species, but to different 

 genera ; and when we have no evidence to go upon at all, one hypothesis may be as good as 

 another. We propose, then, to leave the question of species altogether out of account, and 

 analyse the known foi-ms, in order to discover, if possible, some definite principles of growth 

 other than the mere fact that the forms are encrusting as opposed to brandling. The result 

 has been that, besides the three subdivisions la, \h, \c mentioned above, it is possible, as 

 we shall now see, to distinguish surface markings, with a definite principle of growth under- 

 lying them. 



The principle is as follows : The mounds or eminences which appear on any normally 

 growing thick encrusting or massive form, repeat the shape of the initial stock. Or we may 

 say that the shape assumed by the initial colony of any such Porites will be assumed again by 

 every group of polyps large enough and well enough placed upon the surface of the colony as it 

 grows larger and larger. As illustrations of this, we refer to the following : — 



Pourtales' * splendid figure of P. Florida 6 (see p. 75), which shows a smooth hemispherical 

 mass covered with equally smooth hemispherical eminences. 



Dr. Vaughan's f photograph of P. Porto Rico 5 (p. 65) shows a rough hemispherical mass 

 looking like the bark of a tree, covered with eminences of exactly the same character. 



P. Belize 3 (p. 69, PI. XVII. fig. 7a), which has a long ovate outline, and tends to rise like 

 a rounded angular wave ; the surface is raised all over into eminences of this same shape. 



P. West Indies x. 28 (p. 104, PI. XVII. fig. 11) rises into an almost round-topped conical 

 mass, and the whole surface is covered with vertical round-topped upgrowths. 



P. St. Domingo 1 (p. 66) rises like a peaked mountain mass, and the whole surface is 

 covered with peaks. 



While I admit that these cases are not numerous, they are so clear and exact that we are 

 fully justified in maintaining that they point to the existence of some definite principle of growth 

 in coral stocks, which has never previously been noticed. It seemed at first as if it could be 

 expressed somewhat as follows : that the form assumed by the initial colony is always 

 endeavouring to repeat itself on every suitable surface of the resulting stock. This seems 

 simple enough, and the connection it has with the more comprehensive law of metameric 

 growth, to the discovery of which it led, is obvious. 



The Branching Forms, Divisions 2, 3, 4. — Leaving the astneoid forms, we pass now 



* Florida Reefs, Mem. Mus. Comp. Zool. (1880) pi. xvi. 

 t Bull. U.S. Fish Commission, ii. (1900) p. 289, pi. xxxii. 



