500 HYATT'S REVISION OF THE 



knowledge, the affinities of the Mediterranean forms and of many Pacific forms, with 

 those of the West Indian regions, and the greater number of the species and varieties 

 found there. 



Several authors have remarked the susceptibility of the sponges to physical influence, 

 notably Bronn, Schmidt, Ha^ckel and Miklucho-Maclay. 



Bronn remarks, in his general treatise on the sponges, as follows : — " Man hat bemerkt 

 das die naher an der Oberflache lebenden und den Bewegungen des Wassers, des Sandes, 

 der Steine u. s. w. mehr ausgesetzten Arten dichter von Gewebe sind, als jene aus grossen 

 Tiefen, oder dass sie wenigstens die geschiitztesten Stationeu aufsuchen. Bronn, Klassen 

 und Ordnungen d. Thierr. 1, p. 24. 



Schmidt, in " Spongien d. Adriatischen Meeres," p. 40, considers that a warmer climate 

 increases the deposit of horny matter in the skeleton of Scopalina toxotes. "■ Das sudliche 

 Klima ist dem Uebergange der Sarcode in die Hornfasern giinstig ; beim Verriicken gegen 

 den oberen Theil des Adriatischen Meeres is diese Eigenschaft bis auf den Grad verloren 

 gegangen, welcher als Gattungsmerkmal fiir Scopalina aufgestellt werden konnte." This 

 agrees very well indeed with my own researches, which show that the coarsest skeletons 

 of the same species are in the warmest localities, so far as the genus Spongia are con- 

 cerned, and it may be presumed to be tnie also with regard to the whole group of the 

 Keratosa ; but some species of Chalince have thicker fibres in the northern than in the 

 southern forms, and we cannot therefore apply it to all the fibrous sponges, as a law having 

 no exceptions. 



Miklucho-Maclay, m commenting on the remarks of Bronn and Oscar Schmidt, agrees 

 with them so far as they show the adaptation of the species to its surroundings, and adds, 

 " Nicht bloss die Bewegung des Wassers, sondern auch wahrscheinlich die Temperatur, der 

 Salzgehalt, so wie die chemische Zu.sammensetzung des Wassers sind Factoren welche die 

 Skeletbildung bedingen oder verandern." 



Hajckel also, in his Calcispongiaj, frequently alludes to the modifying influence of physi- 

 cal causes, and all of these authorities view this influence in the same way. They regard 

 the modifications which take place as adaptations of the organization to its surroundings. 

 Whether the modifications are to be viewed as the direct product of the agency of the 

 physical causes themselves, or partly to the action of Natural Selection, is not so clearly 

 expressed by them as might have been wished. 



Similar researches to that in which we are now engaged have been made of late uj^on 

 terrestrial animals by several authors, and notably by Mr. J. A. Allen, of the Museum of 

 Comparative Zoology, Cambridge. The results attained by him are so much more exact 

 and reliable, and so nearly parallel with my own, which are merely preliminary to a fuller 

 consideration of the subject in future parts of this Memoir, that I need make no excuse for 

 the insertion of the following abstract of the influences of environment upon mammals and 

 birds, which has been kindly furnished by him at my special request. 



'• The direct action of the conditions of habitat upon mammals and birds has recently 

 been shown, with reference particularly to those of North Ameiica, to be very strongly 

 marked, resulting in modifications of the general size, of the coloration, and the relative 

 size and form of particular organs. These modifications are clearly local, affecting sim- 

 ilarly, though not always to the same degree, all, or nearly all, the species inhabiting the 

 same areas, so that variations of a somewhat special character become characteristic of par- 



