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Selachia are tooth-clad all over tbe body; in the higher lishes only 
the mouth and pharynx have got teeth, in the amphibians they 
are limited to the jaws and the palate, in the higher reptiles to 
the jaws and as yet are replaced through life; in mammals they 
are confined to the jaws and no more than two tooth-generations 
successively appear. The mammals must be derived from the rep¬ 
tiles ; we have therefore to look for the most primitive among the 
forms with the greatest produetion of teeth, i. e. the Edentata and 
Cetacea. Ofthese the Edentata are the more primitive, because their 
teeth are ever-growing like those of the reptiles, while the teeth of 
the Cetacea have acquired a root; the former accordingly produce 
more substance than the latter. Wherever we see ever-growing 
teeth, we find a primitive condition. Believing them to be derived 
from rooted teeth would oppose the law of reduction. Baume 
maintains the singular point of view, „dass nie ein Thier mit 
immerwachsenden Zåhnen von einem Thier mit Wurzelzåhnen ab- 
stammen kann, wohl aber umgekehrt letzteres aus ersterem. So 
kann zum Beispiel Chiromys niemals von einer der heutigen Lemuren 
åhnlichen Form abstammen.“ (pag. 167). It might seem to be a 
contradietion that the rooted teeth — which are supposed to be more 
reduced as to substance than the rootless, — appear in two dentitions, 
whereas the ever-growing are not replaced. But Baume asserts 
that m reality the whole principle of tooth-replacing is founded on 
a misunderstanding; the milk-teeth are not the predecessors of the 
permanent teeth, but their contemporaries; they are doomed to 
vanish altogether, but are still retained as tiny, early developed 
teeth which in process ot time will entirely fail to appear. _ We 
have thus among the Vertebrates three stages of produetion of 
tooth-substance: 1) Amphibians and Beptiles: constant loss of 
half-worn teeth, over produetion of substance. 2) Ever-growing 
mammalian teeth: unlimited growth, wear and inerease equalise. 
o) Rooted mammalian teeth: The growth is early accomplished, the 
wear is not compensated, but counteracted by the enamel; great 
economy. The reduction is accompanied by a specialisation of the 
