246 NATURAL SCIENCE. 



JUNB, 



transgressed either by the influence of the surroundings or by slow 

 and progressive evohition. These Hmits are so sharply defined 

 that, notwithstanding the great number of species of moUusca 

 founded upon insufficient materials, it has always been easy for me 

 to distinguish one species from another. I would even add that 

 among the twelve or fifteen thousand species I have studied, I have 

 never found intermediate species establishing a gradation from one 

 to another, and I have only recognised in the variability of moUusca 

 a peculiar faculty by which certain species are enabled to assume 

 the form and coloration of the bodies on which they are fixed — a 

 provision through which they can live without danger in localities of 

 different aspect and greater extent. 



" Moreover, I cannot explain to my satisfaction the passage of 

 one species to another, without assuming an abnormal cause, which, 

 suddenly coming into operation at undetermined periods in certain 

 parts of the globe, would change the direction of growth which each 

 species inherits from its birth." 



Palaeontologists, we imagine, will dissent from the latter pro- 

 position most emphatically ; and we may express the hope that Dr. 

 Jousseaume will soon be able to publish some precise details as to 

 the evidence on which his conclusions are based. So far as the 

 shape and ornamentation of the shells are concerned, numerous 

 examples of the most gradual modification of the forms of one 

 geological stratum into those of an immediately overlying stratum, 

 could be cited by almost any student of the Secondary and Tertiary 

 mollusca. 



The Succession of Teeth in the Mammalia. 



It is a familiar fact that, whereas nearly all the lower vertebrates 

 — fishes, batrachians, and reptiles — have an almost unlimited power 

 of reproducing their teeth as occasion requires, the higher vertebrates, 

 or mammals, are never provided with more than one change of teeth 

 during their lifetime. It has also been for a long time well ascer- 

 tained, that the lower mammals (Marsupialia) as a rule exhibit even 

 less change of teeth than the higher (or placental) mammals. There 

 has thus been great difficulty hitherto in explaining the manner in 

 which the mammalian type of dentition became evolved from the 

 primitive constantly-reproduced type such as we observe, for instance, 

 in the crocodiles. Numerous theories have been proposed to account 

 for the apparent anomaly that the lower mammals exhibit less tooth- 

 change than the higher mammals ; while, contrary to the teaching of 

 the great pioneers in Comparative Anatomy such as Cuvier and 

 Owen, the majority of the modern School has long held the belief 

 that mammals originally possessed but one set of teeth and gradually 

 acquired the power of reproducing part of this set once in a lifetime. 



The remarkable investigations of Dr. Willy Kiikenthal, treated 

 in a lecture published last month,' and in previous, more detailed 



1 " Ueber den Unsprung und die Entwickelung der Siiugetierzahne," Jena. 

 Zeitschy. f. Natunv., vol. xx%i , pp. 469-489 (1892). 



