1034 LIFE : OUTLINES OF GENERAL BIOLOGY 



shelter for a strain that is for some reason or other conservative, 

 and not, for the time being, susceptible to variational stimulus. 



Besides observations, Kammerer made numerous interesting 

 experiments. Thus lizards subjected for a long time (up to two 

 years) to high temperature became dark in colour, more rapidly in 

 conditions of drought. Humidity made the colour lighter; humidity 

 combined with lowered temperature did not accelerate the change, 

 but carried it further. It was not possible to expose the lizards to 

 more natural sunshine, for they already enjoyed the maximum; but 

 conditions of shade induced some lightening of the colours. In the 

 case of the freshwater crab (Potamon fluviatilis) , shutting off direct 

 illumination resulted in a change of the colour of the upper surface 

 from the normal brown to black. On a white ground the lizards 

 become lighter, on a black ground darker. A very important con- 

 clusion, bearing on the whole process of organic evolution, is that 

 the progeny of modified individuals exhibit similar modifications 

 when they are reared, not in the new conditions, but in inter- 

 mediate (mittleren) conditions, where control-specimens are unaf- 

 fected. When the offspring of the modified individuals are reared in 

 the new conditions, they go further in the same direction, if that is 

 possible. 



The black variety of Lacerta serpa {mellisellensis) becomes 

 slightly lighter when kept in cool humid surroundings, but the 

 offspring kept in the same conditions become as markedly light as 

 the original form. We must notice, however, in regard to Kam- 

 merer's conclusion, that it will be necessary to take account {a) of 

 the longer time of exposure to the new conditions, and {h) of the 

 influence of these on the plastic young stages. Even the reproductive 

 organs within the parent may be affected, as in Agar's water-fleas 

 (see section on Transmissibility of Modifications). 



Summary. — ^There are hundreds of Dalmatian islands and each 

 has its own facies of lizards. All are descended from Lacerta serpa 

 and L. fiumana or the common ancestor of these. The variations 

 that are at present continuing can be grouped for the most part 

 as alternatives in opposite directions — expressing increased and 

 decreased activity, size, darkening, and so forth. The established 

 or permanent strains have their analogues, if not antecedent stages, 

 in individual modifications which are observed to occur. There is 

 in many cases a definite sequence in the fixation of a new departure, 

 e.g. in the adult males first. Arrests and reversions also occur. 

 The results of experiment corroborate what is suggested by observa- 

 tion — the fundamental importance of environmental diversity. 

 Whether the environmental peculiarity produces a direct modifica- 

 tion which is entailed on the offspring and carried further in subse- 

 quent generations (as Kammerer believes), or whether it acts as a 

 liberating stimulus on germinal variability, must remain, for us, a 



