PRINCIPLES OF ANIMAL BIOLOGY 49 



The pelvic girdle is fused into a single bone except in young birds. 

 The ilium is the broad flat part above. The pubis is the slender curved 

 rod at the lower margin of the girdle, behind the acetabulum. It is 

 partially separated from the rest of the girdle by a long cleft. The 

 ischium lies above this cleft and below the large opening behind the ace- 

 tabulum. Observe the femur in the thigh, and the tibio-tarsus [in the 

 lower leg. The fibula is a slender bone fused to the tibio-tarsus near its 

 upper end. The tar sals are not present as distinct bones, some of them 

 being fused with the tibia and some with the metatarsals. The latter 

 together with the fused metatarsals form the tarso-metatarsus of the foot. 

 Note that it is a triple bone, the second, third, and fourth metatarsals 

 being fused. At the proximal end they are fused with some of the tarsal 

 bones. The first metatarsal is a separate bone applied to the inner edge 

 of the tarso-metatarsus at its distal end. There are four digits. The 

 distal phalanx of each is modified for the support of the claw. The first 

 digit points backward, the second, third, and fourth forward. How many 

 phalanges in each digit? 



Draw two of the limb skeletons studied and label fully. 



Prepare a chart comparing the limbs of man, the frog, and the pig- 

 eon. In the first column place the names of all the bones found in the 

 limb skeleton of any of the animals studied; in three other columns state 

 whether the bone in question is in any striking way modified from the 

 hypothetical ancestral condition (3a, 36), such as fusion with another 

 bone, reduction in number, etc., in the three animals named. 1 If not 

 so modified, leave the corresponding place in the chart blank. What does 

 this chart, when completed, show? 



C. MODIFICATION OF LIMBS IN EVOLUTION 



Animals possessing homologous structures, no matter how different 

 those structures are in the adult, are believed to be related to one another. 

 That is, they are believed to have descended from a common ancestor at 

 some more or less remote time. If this belief is well founded, these 

 structures must have become modified from the ancestral condition. 



While it is easy to demonstrate, as has been done above, the similarity 

 of origin of such homologous structures, it is usually impossible to trace 

 the evolutionary changes by which the similar structures became different. 

 These changes can be certainly known only from the fossils of animals 

 from various points in the line of descent from the common ancestor, and 

 such fossils are usually wanting. For this reason the stages of modifica- 

 tion in the frog and bird are not thoroughly known. In the horse, how- 



1 The student should have acquired in this exercise the ability to name most of the 

 bones found in the limbs of any vertebrate. At the option of the instructor, an addi- 

 tional form, such as the horse, may be introduced to test this ability. 

 4 



