82 Instinct. 



the constant results produced by it. All life does 

 not build up birds any more than all insects build 

 honey-comb. Life has as many specific characters 

 as there are distinct forms of living beings. 



We are not now discussing the question how 

 these differences came to be, but simply call atten- 

 tion to their manifestations. We have life a con- 

 stant quantity, as we should express it in mathe- 

 matics, and we have joined to this a vast number of 

 variables which give us the forms of life as manifest- 

 ed in distinct kinds. These kinds have not only 

 life in common, but even the variables have some- 

 thing in common, so that the kinds can be arranged 

 into groups according to the similarity of these va- 

 riables, giving us GENERA, FAMILIES, CLASSES, and 

 finally two kingdoms, animal and vegetable, founded 

 mainly upon the variable, saisation. 



We do not wish here to be understood as en- 

 dorsing the view that these variables are constantly 

 changing or liable to change. We only speak of 

 them as variables because they are the cause of dif- 

 ferences in forms, all due to one great underlying 

 principle which we call life, which no one fully un- 

 derstands, but the distinctive phenomena of which 

 every intelligent person understands as well as he 

 does the phenomena of gravitation. We regard 

 these variables as the same in kind as those that 

 give rise to the different kinds of matter, or at least 

 strongly analogous to them. We have the generic 

 notion of matter gained from certain properties that 

 must be present to give the notion of matter at all, 

 and then the variables that give the different kinds 



