FOSSIL VARANIDAE AND MEGALANIDAE. 
481 
Lydekker, both allude to Varanus as already native of India 
in the Pliocene, from whence yet in the Neogene they must have 
migrated, approximately in the direction given by Arldt, 1 2 further to 
Australia, where however they are only known from the Pleistocene onward. 
In connexion with the geographical distribution it is of highest im¬ 
portance to consider the phenomenon of variation and number of 
species. As has been stated, it is in South-Eastern Asia that most species 
are met with. The conclusions to be drawn from this fact appear worthy 
of examination. Dr. Szombathy 2 in his paper on fossil Potamon refers to 
the question of variation in the following terms: «Wenn die Urheimat einer 
Tiergruppe dort zu suchen ist, wo die dazu gehörigen Arten den höchsten 
Grad ihrer Variabilität erreichen, so muss das Zentrum der Entwicklung der 
Potamon- Arten im indischen Faunengebiet vermutet werden.» This hypo¬ 
thesis, in the present case, is supported by the palaeontological fact of the 
oldest veritable Potamon , according to Arldt, having be obtained from 
Hindustan. This theory seems correct from the b i o 1 o g i c a 1 point of view 
as well, inasmuch as the characters of a species of primitive degree, that is to 
say yet only slightly differ enciated, are always more liable to 
change, their variability is therefore greater. The occurrence 
of this phenomenon justifies in many cases the supposition of these 
species originating from the territory where the variation is more strikingly 
apparent, whilst in the course of further migration their morphological 
characters had the possibility of developing and thus far from 
their land of origin, acquired stability. 3 This may be the case 
with L. viridis and L. agilis, mentioned by Prof. Méhely, and referred to 
above, offering an example all the more comprehensible as two immediately 
related forms are in question, the lack of différenciation 
between them eo ipso pointing to the primitive nature of the respective 
forms. Convergence in such a direction and degree, is scarcely 
possible. It must be remembered nevertheless that the variation of certain 
forms often permits to conclude not to their primitive or ancient charac¬ 
ter, but on the contrary to a period of development where a new variation 
rises, or as De Vries says : a period of mutation, by which these forms 
abandon the stricter limits of the species. As may be judged a defi¬ 
nition from the biological point of view is most difficult 
and claims great cautiousness, Therefore Dr. Bzombathy’s opinion, 
1 L. c. 
2 Die Tertiären Formen d. Gattung Potamon (Telph.) u. ihre paläarkt. Naehk., 
Ann. Mus. Nat. Hung., Vol. XIV, Budapest, 1916, p. 418. 
3 This however may only be established as a possibility and not as a faot. 
