Vol. II, Pt. I] VAN DENBURGH— GALAPAGOS TORTOISES 289 



Examination of these charts shows four kinds of variation 

 in shape : 



1. Variation with age. 



2. Variation with sex. 



3. Variation with distribution. 



4. Individual variation. 



1. Variation zvith age. — Young tortoises of all races are 

 similar in shape. The racial characteristics become evident 

 only after the tortoises have attained a considerable size. The 

 differences between the young and adult are more marked in 

 the male than in the female, and in the so-called saddle-backed 

 than in the non-saddle-backed races. In other words, the 

 young are all more or less dome-shaped, the elevation and 

 constriction of the anterior portion of the carapace in the 

 saddle-backed races being acquired later in life. 



In the 3^oung, the front height averages less ; while the 

 middle height, the height to marginals, the curved length, the 

 length of plastron, and the straight width average more than 

 in the adult. The plastron is flat, notched posteriorly, and 

 lacks the posterior knob-like thickenings which later develop 

 in the males. 



2. Variation zvifh sex. — In the purely dome-shaped races, 

 such as that of Indefatigable Island, there is but little difference 

 in shape between the sexes. In the intermediate races, such as 

 those of Tagus Cove, Iguana Cove, James and Chatham 

 Islands, the female retains the high-backed, low-fronted cara- 

 pace; while in the male the anterior portion becomes elevated, 

 and the back, in consequence, appears flattened. In the saddle- 

 backed races, the males have the anterior portion raised still 

 higher, so that it sometimes is higher than the middle of the 

 carapace, and the first costal plates with the corresponding mar- 

 ginals appear as though pressed inward toward the median line. 

 The females of these races show this elevation and constriction, 

 but in a lesser degree. 



In adult males the plastron often is quite concave, is shorter 

 than in females, and is thickened at its posterior extremity into 

 broad knob-like masses. These tumefactions seem to be pecu- 

 liar to the males except in one race. In Testudo giintheri of 



