chap, xi The Elements of Structure 173 



ratio to structural or morphological simplicity. Thus the 

 physiologist makes most progress when he seeks to under- 

 stand animals with many parts, for there he can find a large 

 number of units, all as it were working at one task. The 

 life of a Protozoon is more manifold and complex than that 

 of any unit from a higher animal, just as the daily life of 

 the savage — at once hunter, shepherd, warrior — is more 

 varied than ours. 



Already it has been recognised that every many-celled 

 animal begins its life as a single cell, — as an egg-cell with 

 which a male element has united. Every Metazoon begins 

 its life as a Protozoon, no matter how large the animal, 

 for the whales arise from ova "no larger than fern-seed,"* 

 no matter how lofty the result, for man himself has to begin 

 his life at the literal beginning. The fertilised egg- cell 

 divides and re-divides, its daughter- cells also divide, the 

 resultant units are arranged in layers, clubbed together to 

 form tissues, compacted to form young organs, and the 

 result is such a multicellular body as we possess ; but while 

 this body-making proceeds, certain units are kept apart, in 

 some way insulated from the process of growth, to form the 

 future reproductive elements, which, freed from the adult 

 body, will begin a new generation. Back to the beginning 

 again every Metazoon has to go, and if we believe that the 

 Protozoa are not only the simplest, but also represent the first 

 animals, we have here the first and perhaps most important 

 illustration of the fact that in its development the individual 

 more or less recapitulates the history of the race. The 

 simplest animals are directly comparable with the repro- 

 ductive cells of higher animals, but the divided cells of the 

 ovum remain clubbed together to form a young animal, while 

 the daughter-cells of a Protozoon separate from one another, 

 each as a new life. 



The gulf between the single-celled and many-celled 

 animals is a deep one, but it has been bridged. Otherwise 

 we should not exist. Traces of the bridge now remain in 

 what are called " colonial Protozoa," which, however trouble- 

 some to those who like crisp distinctions, are most instruc- 

 tive to those who would appreciate the continuity of the 



