190 



TJie Study of Animal Life part hi 



occur in some worm-types (some flukes, threadworms, etc.), 

 and as high up in the series as Tunicates ; while arhong 

 plants analogous alternations are very common, e.g. in the 

 life-cycles of fern and moss. 



Fig. 35. — Diagram of a hydroid colony, some of the individuals of which have 

 been modified as swimming -bells or medusoids ; one of these has been 

 liberated. 



3. Historical, — In the seventeenth and eighteenth cen- 

 turies, naturalists had a short and easy method of dealing with 

 embryology. They maintained that within the seed of a 

 plant, within the ^gg of a bird, the future organism was 

 already present in miniature Every germ contained a 

 miniature model of the adult, which in development was 



