36 Mr Gadow, Orihogenetic Variation, etc. [Nov. 14, 



When there are 8 median scutes the 7th is generally the 

 smallest, being in a half squeezed-out condition. The same 

 applies to the 5th scute, especially where there are only 7 or 6 

 median scutes in all. Again, when the 5th median scute is much 

 reduced and irregular, the 4th pair of costals exhibits all the 

 stages from the normal size to the tiny, almost completely sup- 

 pressed, vestige of a scute. 



The normal shield possesses 6 median (including the so-called 

 nuchal) and 5 pairs of costal scutes. 



Of the total of 56 specimens not less than 43 are abnormal, 

 = 76-6%. 



Of the 41 newly-hatched not less than 38 are abnormal, 



= 92 " 7 7o. 



The percentage of abnormalities is 4 to 5 times as great in the 

 newly-hatched as in the adult, and it decreases gradually from the 

 smaller to the larger and very large specimens. 



We have no reason whatever to assume that those little turtles 

 which were born with irregular scutes were doomed to perdition, 

 while only the normal specimens were predestined to live and to 

 propagate the race. Such an assumption is contradicted by the 

 fact that not less than 20 to 25 °/ of mature turtles are wrong in 

 their shields and do very well indeed for all we know to the contrary. 



A comparative anatomical study of the carapace and shield of 

 the Chelonia shows that there existed originally Tortoises with 

 8 median and 8 pairs of costal and dermal bones, the latter corre- 

 sponding in number and position with the epidermal scutes. 



The present abnormalities are atavistic reminiscences, and 

 most of the individuals seem to grow out of those irregularities, 

 and this reduction or squeezing-out of some of the scutes proceeds 

 in a very regular way. It begins with the 7th and 5th median, 

 and the 5th or 4th pair of costal scutes respectively, and ends, in 

 the " Loggerhead," with the suppression of the originally 2nd pair 

 of costal scutes. 



This process is an instance of a widely-spread law, namely, that 

 the number of a serial set of organs or parts has a tendency 

 towards reduction in numbers, while the remaining parts are 

 better developed, more neatly finished, and consequently become 

 more effective. 



Our little turtles start with many, with at least 22, dorsal 

 scutes (leaving out the marginals), and they reduce them to 16. 

 A further reduction to 14 (by suppression of the originally first 

 pair of costal scutes) occurs in Chelone imbricata and in the 

 majority of the Tortoises. Suppression of the nuchal or first 

 median scute reduces the total number to 13, a condition pre- 

 vailing in most of the side-neck -hiding Tortoises and in some 

 species of Testudo. 



