EVOLUTION 715 



same thing even more strikingl}'. Those of Carniola in Yugo- 

 slavia and Kentucky in the United States are both in hmestone, 

 and the animals in them — urodeles, fishes and invertebrates- 

 are very similar, with characteristic modifications such as blind- 

 ness, but their structural afiinities are with the animals of the 

 continent in which they are situated. 



5. The student w^ho has read as far as this chapter will have 

 realised that a comparative treatment of morphology is only 

 possible because to a great extent different animals are built 

 on the same plan. We can recognise the cell in almost all animals 

 and most plants. The nephridium, a hollow ingrowth from the 

 exterior ending in a tuft of cilia, can be seen in Platyhelminthes, 

 Annelida, Rotifera, ^lollusca, and Branchiostoma. The general 

 plan of the vertebrate skeleton is similar throughout the group, 

 and within a class there may be only small differences in shape 

 in the bones of corresponding positions (Fig. 550). Many animals 

 contain apparently functionless vestigial organs, such as the 

 appendix and dermal muscles of man, which seem to represent 

 structures which in other animals, such as the rabbit and horse 

 in the examples given, have physiological value (Fig. 551). 

 None of this by itself constitutes evidence for organic evolution, 

 for there is equally a comparative morphology of things like motor 

 cars which also may contain vestigial structures, although the 

 theory is inapplicable to inanimate things, but taken together 

 with the fact that, so far as we know, new individual animals 

 only arise by reproduction from pre-existing animals, whereas 

 each new motor car is made de novo, it does. 



6. Similarly, comparative physiology supports the theory of 

 evolution, and in general the closer the structural relationship 

 between two animals the greater is the resemblance between the 

 chemical processes that go on within them. In a few instances 

 comparative biochemistry has added weight to suggestions 

 made on rather slender evidence by anatomists. There is, for 

 instance, slight structural resemblance between chordates and 

 echinoderms, and of all the animals which have been examined 

 the echinoderms most resemble chordates in the chemistr\' of 

 their muscular contraction. 



7. Even when the adults of two species of animal resemble 

 each other very little, their embryos may do so much more 

 strongly. This, sometimes known as Von Baer's Law, or the 

 biogenetic law, was known as a generalisation some years before 



