82 THE LIVING WORLD. 



his body. The development of animals has always 

 been thus a differentiation. But we must bear in 

 mind that only the undifferentiated can become 

 differentiated, and it is plain that after an animal 

 has once become differentiated in any direction the 

 possibilities for further differentiation become imme- 

 diately limited. Let us notice a single familiar ex- 

 ample in illustration. At one time in the history of 

 the horse family the possession of five toes on each 

 foot was a common character. From a five-toed 

 condition a large number of lines of descent were 

 possible. But the actual animals entered upon a 

 line of progress which resulted in the successive loss 

 of four of their toes. Now with each step in this 

 progression the possibilities for further development 

 became limited. The animal with four toes \vas 

 forever debarred from all lines of progression that 

 required five, and the horse of to-day, having only 

 one toe, has practically ended the possibilities of de- 

 velopment in this direction. This one toe he must 

 of course retain ; and since it is a law of biology that 

 an organ once lost is never redeveloped, any further 

 differentiation of the toe of the horse is impossible. 

 Now the same principle is true everywhere. As 

 soon as the first step in any line of differentiation is 

 taken, the possibilities for further development be- 

 come immediately limited. 



The most undifferentiated of all types of multi- 

 cellular animals is the Gastraea. In this simple 

 structure there are possibilities of an immense 

 amount of modification, simply because it has 

 developed no structure of its own. But just as 



