No. 564] THE FIXATION OF CHARACTER 



71J 



of teeth, of vertebrae, of digits, of aortic arches, of brain 

 lobes, of cranial nerves and of countless other structures 

 is very conservative and is characteristic of large groups 

 of animals. In the plant kingdom the fixity of number 

 is even more noticeable. Throughout gymnospermous 

 plants the number of sporangia to a sporophyll, in both 

 the male and the female cones, varies but slightly. The 

 two great groups of angiosperms, the dicotyledons and the 

 monocotyledons, can be separated on but one constant 

 character, the number of cotyledons in the embryo. The 

 numerical plan of the flower in both series is also very 

 constant, being almost invariably four or five in the dicoty- 

 ledons and three in the monocotyledons. Most angiosperm 

 families, or genera, at least, have a characteristic number 

 of sepals, petals, stamens and carpels, which is of great 

 importance in classification. Similar instances could of 

 course be multiplied almost indefinitely. 



Conditions of relative position and of insertion of parts 

 are also notably conservative and of value in determining 

 relationships. In the higher invertebrates, for example, 

 the nerve cord is always ventral to the digestive tube and 

 chief blood vessels, whereas in the vertebrates it is in- 

 variably dorsal. The mass of the liver may be disposed 

 almost anywhere, but its attachment is always on the ven- 

 tral side of the digestive tube. The source of the nerve 

 supply to many organs is exceedingly slow to change and 

 is of much importance in determining the primitive posi- 

 tion of structures which have been moved from their 

 original situation. Among plants, the relation of bud to 

 leaf is very constant, and the particular relative positions 



formed wood elements, and of parenchyma cells and ves- 

 sels are very characteristic for each of the main groups 

 of vascular plants. The degree of coalescence between 

 the members of the same floral circle and the method of 

 insertion of each of the floral circles upon the axis or upon 

 one another are admitted to be of the greatest diagnostic 

 value. 



