ORTHOGEXETIC VARIATION IN THE SHELLS OF CHELONIA. 221 



less transverse series of epidermal scutes, while the constituting elements of the dermal 

 armour after having been welded into the formation of the carapace, remain compara- 

 tively constant. 



But to return to Thalassochelys caretta. 



This table shews that the abnormalities are 4 to 7 times as common in the new- 

 born as in the mature specimens, and that their frequency decreases from the smaller 

 to the larger and very large specimens. 



We have no business whatever to assume that our little Turtlets which are born 

 with irregular scutes are therefore doomed to perdition, while only those born with 

 the normal number are predestined to live and to propagate the race. Such an 

 assumption is at once contradicted by the fact that no less than 14 °/ of large 

 turtles are wrong in their scutes, and do, or did very well, for all we know to the 

 contrary. 



Of course comparatively very few individuals of a brood of several dozen Turtlets 

 reach maturity, but they meet with their death through Sea-birds, Crocodiles, Sharks, 

 and similar enemies, which in all probability swallow them regardless of the number 

 and disposition of their victims' scutes. 



These abnormalities are atavistic reminiscences, and most of the creatures grow 

 out of these irregularities by the reduction or squeezing out of certain of the scutes. 

 Why this suppression shoidd take place in the region of the original 7th and 5th 

 transverse series of neural and costal scutes I do not profess to know. 



We have here an instance of a widespread evolutionary 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, are more neatly finished and can 

 therefore be made more highly effective. 



The eight or nine gill-supporting visceral arches of the early Elasmobranchi are 

 reduced to five or four or even less gill-bearing arches in the Teleostomi with highly 

 finished pectinated gills. The multiserial fish-fin has been changed into our penta- 

 dactyle limb. The innumerable skin-denticles, the shagreen of Elasmobranchs, have 

 produced the dermal plates and bones of higher creatures. Of the six primitive arterial 

 arches there remain only three, or may be two, etc., etc. 



Our Turtlets start with many, with at least 24 dorsal scutes (leaving out the 

 marginals), and they reduce them to 16. In other genera the reduction has advanced 

 to 14, to 13, and individually even to 12. 



This means onward development. The ideal, the goal for the young Caretta is 

 the possession of a 16 scuted shell. Those which start with 24 perhaps never reach 

 the ideal, but this failure does not seem to hurt them, natural selection remains 

 iudifferent. Others start with 22, 21, 20, 19 or 18 scutes, and the latter individuals 

 are rather common in the newly hatched stage, and all of these seem to reach the 

 goal. Lastly there are some precocious Turtlets of the same brood, which start with 

 the right number of 16 scutes, but if they devote their superfluous energy to some- 

 thing better than the making of tortoise-shell we do not know. 



Anyhow this is onward development. These variations from the normal type all 

 lie in the direct line of descent, and the more serious the variation, the further back 



