26 THE EVOLUTION THEORY 



arises before the rest of the body is complete, and how the blood begins 

 to circulate ; in short, there was disclosed all the marvel of develop- 

 ment to which we are now so much accustomed, that we can hardly 

 understand the sensation it made at that time. 



Later on, attention was turned to the development of Fishes and 

 Amphibians (Agassiz and Vogt, later Remak), then to that of the 

 Worms (Bagge), of Insects (Kolliker), and gradually the development 

 of all the groups of the animal-kingdom — from Sponges to Man — was 

 so thoroughly investigated that it almost seems to-day as if there 

 could not be much that is new to discover in this department. This 

 impression may indeed be true as far as the less complex processes 

 and the more obvious questions are concerned, but it is impossible 

 to predict what new problems may confront us, whose solution will 

 depend on a still more detailed study of development. 



As embryology is a science of the nineteenth century, so also 

 is histology, the science of tissues. Its pioneer was Bichat, but its 

 real foundations were not laid till Schwann and Schleiden formulated 

 the conception of the ' cell,' and proved that all animals and plants 

 were composed of cells. What Oken had only guessed at they now 

 proved, that there are very minute form-elements of life which build 

 up all the parts of animals and plants or produce them by processes 

 of secretion. New light was thus shed on embryonic development, 

 and this gradually led to the recognition of the fact that the egg, too, 

 is a cell, and that development depends on a cell-division process in 

 this egg-cell. This led further to the conception of many-celled and 

 single-celled organisms, and so on to many items of knowledge to 

 speak of which here would carry us too far. 



For it is not my intention to attempt a complete review of the 

 development of biology in the nineteenth century, or even in the 

 period which we have mentioned as devoted to detailed research ; 

 it is rather my desire to convey a general impression of the enormous 

 extent and many-sidedness of the progress that was made in this 

 time. Let us therefore briefly recall the entirely new facts which 

 were brought to light in this period with regard to the reproduction 

 of animals. Asexual reproduction by budding and division was 

 already known, but parthenogenesis is a discoveiy of this period, 

 and so also is alternation of generations, so far-reaching in its 

 bearing on general problems. It was first observed (18 19) by 

 Chamisso in Salpa, then by Steenstrup in Medusaa and trematodes, 

 and was later made fully clear in its most diverse forms and relations 

 by the researches of Leuckart, Vogt, Kolliker, Gegenbaur, Agassiz, 

 and other illustrious investigators. Reproduction by heterogony, too, 



