"Another colour modification which Dr. Kammerer states that he obtained by raising the temperature is the assumption by the female of the typical _Lacerta muralis_ of the bright red colour of the lower parts which often distinguishes the male from the female, and which was not shown by the individuals of the latter sex kept by him under normal conditions. He quotes various authorities to show that the lower parts are never red in the females, but he has omitted to consult others who say the contrary. Thus Bedriaga (1878 and 1879) remarks that a so-called var. _rubriventris_ of the typical wall lizard has the lower parts red in both sexes."[29] In reading such papers as those of Semon or Kammerer the thought uppermost in my mind is that to multiply illustrations of supposed transmission of acquired characters is of little use until some one example has been thoroughly investigated. If we had certain assurance that even a single unimpeachable case could be repeated at will, the whole matter would assume a more serious aspect. If, for instance, Kammerer were able to show us _Alytes_ males with horny pads on their hands, it would be something tangible; still more, if the experiment were repeated by others until no doubt remained that the offspring of _Alytes_ which had bred in water for some three generations did acquire these pads and that they could transmit these novelties to descendants raised in normal conditions. Till evidence of this kind is published by at least two independent observers investigating similar material, I find it easier to believe that mistakes of observation or of interpretation have been made than that any genuine transmission of acquired characters has been witnessed. Meanwhile there is no denying that the origin of adaptational features is a very grave difficulty. With the lapse of time since evolutionary conceptions have become a universal subject of study that difficulty has, so far as I see, been in nowise diminished. But I find nothing in the evidence recently put forward which justifies departure from the agnostic position which most of us have felt obliged to assume.[30] APPENDIX TO CHAPTER IX. Professor G. Klebs, as is well known to students of evolutionary phenomena, has for several years been engaged in investigations relating to the inheritance of acquired characters. In his many publications on the subject the issue has always been represented as more or less uncertain. Desiring to know how the matter now stands according to Professor Klebs' present judgment I wrote to him asking him to favour me with a brief general statement. This he most kindly sent in a letter dated 8th July, 1912. As such a statement will be read with the greatest interest by all who are watching the progress of these studies I obtained permission to publish it as follows: 8. Juli 1912