1903.] Evolution of the Colour-pattern in Lizards. 109 



" Evolution of the Colour-Pattern and Orthogenetic Variation in 

 certain Mexican Species of Lizards, with Adaptation to their 

 Surroundings." By HANS GADOW, F.R.S. Received May 22, 

 -Read May 28, 1903. 



[PLATES 35.] 



I. Introduction. 



The late Th. Eimer* has shown that the variations of the colour- 

 pattern of the European Lacerta muralis proceed each on definite lines 

 until a certain stage is attained which is characteristic of the race, or 

 variety, in question. This mode of development he has termed ortho- 

 genetic. As a rule, the successive changes in the colour-pattern of lizards, 

 are as follows : First longitudinal stripes, then dissolution of the stripes 

 into spots, then confluence of the spots in a transverse direction so 

 that a cross-barred pattern is produced, lastly a uni-coloured appear- 

 ance. There are some lizards which pass through the whole scale 

 during their growth from youth to old age. Others again stop short 

 at perhaps the second, or even at the first stage. This phenomenon 

 of arrested development he has termed cpistasix ; genepistasis when 

 applicable to a whole association of individuals, be such a community 

 a local race, representative of a sub-species, or of the dignity of specific 

 or even higher rank ; ontepistasis when the same phenomenon applies 

 only to individuals, for instance, if a specimen of a normally cross- 

 barred kind should stop short at the spotted stage. Again, there are 

 precocious individuals, races and species, which, instead of passing 

 through the whole scale, begin already with the second, or another 

 later stage. Moreover, there are new and further departures which, 

 leading beyond the normal condition, are liable ultimately to accu- 

 mulate, or to consolidate into the formation of new races, &c. All 

 these changes are, as a rule, initiated, at least they are earlier and 

 more frequently visible in the males, while the females often remain 

 at a lower, more juvenile, level. Lastly, these changes begin near the 

 tail and proceed wave-like forwards, so that, for instance, the shoulder 

 region may be still in one stage, while the lumbar region has already 

 assumed the final aspect. This is Eimer's law of postero-anterior 

 development, and since these successive changes, like successive waves, 

 pass not only over the bodies of individuals of one kind, but also over 

 species and genera (provided they be arranged as striped, spotted, 

 barred and uni-coloured forms), Eimer suggested the term undulatory 

 development. 



* " Untersuclmngen ueber das Variiren der Muuereidechso, cin Beitrag zui- 

 Theorie von der Entwicklung aus constitutionellen Ureachen, sowic zum Darwin- 

 ismus," * Arch. f. Xaturg.,' vol. 47 (1881), pp. 239-517, plates 1315. 



