222 ORTHOGENETIC VARIATION IN THE SHELLS OF CHELONIA. 



it points. Moreover the changes necessary to turn any given variation into another 

 one less abnormal until ultimately the normal condition is reached, are not erratic 

 but stand in strict correlation with each other and proceed strictly on definite lines. 

 I therefore call this kind of atavistic variation orthogenetic. 



Of course there is no proof of what I have tried to explain. Comparative anatomy 

 and common sense tell us it is so. But common sense is not evidence in a sceptical 

 court. The only way of proving the correctness of the view explained in this paper 

 would be to take a number of abnormal turtlets and to watch, while they are growing 

 up, if and how they mend their irregular shells and become normal. 



EXPLANATION OF PLATES XXIV. and XXV. 



The numbers enclosed in circles correspond with the numbers of the specimens described 

 in the list which commences on page 208. All the figures relate to young individuals of the 

 marine Chelonian Thalassochelys caretta. 



In the complete drawing reproduced in Fig. 18, PI. XXV., attention may be drawn, apart 

 from the scutes of the carapace, to the claws on the paddles and the unusually prominent 

 tubercle in the nape of the neck. The figure is drawn to natural size. 



