Development of the MammaUan Phylum. 375 



predaceous marsupials, and predaceous placental mammala. 

 It consequently follows from vny line of argument that a 

 phylogenetic connexion between the forms in question on the 

 basis of the dentition is absolutely inadmissible. 



The question as to the origin of the Mammalia we now 

 answer in the following way. The ancestors of the Mammalia 

 were not theromorphons reptiles, as is usually supposed, but 

 primeval forms (from which indeed the Theromorpha may 

 likewise have originated) living during the Palaeozoic period, 

 with a but little specialized dentition, which still consisted of 

 uniform conical teeth. From these there were developed in 

 the first instance mammals with a multituberculate dentition. 

 Many suggestions may be made as to the causes which 

 may have brought about the origin of the Mammalia. The 

 statements of Haacke * on this point sound quite plausible. 

 According to this writer the mammals, which are warm- 

 blooded in contradistinction to the reptiles, which have an 

 alternating temperature, can only have originated at a time 

 when the temperature underwent an appreciable and perma- 

 nent cooling ; and it is stated that this probably took place 

 during a cold period, which geologists term the Permian 

 (?) Glacial epoch. With the acquisition of a higher tempe- 

 rature for the blood, the development of a bad conductor of 

 heat, in the shape of the hairy coat f, became necessary ; and 

 to this was added the formation of sebaceous glands to grease 

 the hairs, and sweat-glands to regulate the temperature of the 

 body. 



Moreover, in connexion with the lowering of the tempera- 

 ture came the incubation of the ova, for the young had now 

 to be hatched by means of the mother's own bodily heat. In 

 relation with this we have the formation and further develop- 

 ment of the incubatory apparatus, such as we still see it to- 

 day in the case of the oviparous Monotremata. 



We now come to the second part of our subject, that of 

 the development of the mammalian phylum. The existing 

 mammals are divided into three subclasses — Monotremata, 

 Marsupialia, and Placentalia. The bodily structure of the 

 still oviparous Monotremata, although variously modified in 

 consequence of special adaptation, exhibits such primitive 



* Ilaacke, " Ueber die Entstehung des Siiugetiers," Biol. Centralbl, 

 1889, p. 8. 



t In a paper which will shortly be published, and which has been 

 •worked out under my direction, it will bo proved by Herr Komer, one of 

 my students, by means of embryological investigations, that the dermal 

 armature of the armadillos is a secondary acquisition, and that in their 

 original condition these animals were provided with a hairy coat. 



