Classification of the Asconidge. 351 



the system given by Hackel in his " Prodroraus eines Systems 

 der Kalkschwiimme '' *, where the principle of classification 

 by external characters was carried to its furthest logical con- 

 clusion. Many of the genera were based simply upon young 

 forms, such as OlyntJius and Olynthium. The species blanca 

 is a good illustration of this. Miklucho-Maclay f, in his 

 description of this species, gave admirable figures to show 

 the changes of form passed through in the growth of the 

 young " Olynthus " into the reticulate mass formed by the 

 large colonies ; yet this species figures under no less than 

 four genera, and might easily have figured under more still. 

 It might be urged that, since Hackel himself fully recognized 

 the artificial nature of his classification, it is hardly fair to 

 subject it to criticism ; but, on the other hand, it might be 

 replied that it is hardly fair to push a principle to an abso- 

 lutely ridiculous extent and then point to the result as a proof 

 of the badness of the principle in question. Contracted and 

 partly contracted forms, again, play an even larger part in 

 the classifications not only of Hackel but of others also. No 

 one, I think, will now dispute the statement that Hackel's 

 genus Prosycum was founded on individuals with closed pores, 

 and his genera CUstolynthus, Sycorrliiza^ Auloplegma^ &c. on 

 specimens with closed oscula. But the height of absurdity 

 is reached when we find a whole order of calcareous sponges, 

 Metrosyca, with two families and several genera, founded by 

 Hackel on specimens in which different parts of the colony 

 were in different states of contraction. Similarly I was able 

 to show X that Hackel's different varieties of Ascetta clathrus 

 were based on specimens in different degrees of contraction, 

 and that the form which he named Ascetta mirabilisj in order 

 to express his astonishment at finding two varieties united in 

 one specimen, was simply a colony which was in parts more, 

 in parts less, contracted. Yet it is this form which PolejaefF§ 

 regards as proving that the guidance of external differences 

 in classification is very uncertain. It was on contracted 

 specimens of clathrus^ or perhaps of coriacea^ that Gray 

 founded his genus GlatJirina^ a genus which nevertheless, as 

 I shall try to show, the laws of priority oblige us to retain. 

 Contractility is so marked a feature of a large section of the 

 Ascons that its being overlooked almost entirely by Hackel, 

 and by many others who came both before and after him, is 



* Jen. Zeitschr. v. (1870) pp. 236-254. 



t " Beitrage zur Kenntniss der Spougien, I.," Jen. Zeitschr. iv. (1868) 

 pp. 221-232, Taf. iv., v. 



X Quart. Journ. ftlicr. Sci. n. a. xxiii. p. 490. 

 § Loc. cit. p. 4. 



26* 



