REPRODUCTION. 



established forms by heredity and the arising of new forms by varia- 

 tion. The main points of discussion are how, when, and where the 

 physiological phenomena of heredity and variation, leading ulti- 

 mately to evolution, set in. While the discussion of these problems 

 has been going on for centuries past, a great stimulus for approach- 

 ing them in a more rational way has been given by Darwin with the 

 publication of his views of the "Origin of Species." Having observed 

 the great variety of forms produced by breeders of. animals and culti- 

 vators of plants, through artificial selection, he was led to assume 

 that the natural selection has been the cause of the multiformity of 

 animals and plants in nature. It was particularly plausible to accept 

 this view of a gradual development of a new species, if there was 

 taken into consideration the needed adaptation to dominating cir- 

 cumstances; the constantly taking place in nature of the struggle 

 for existence , with its consequence of the survival of the fittest; and 

 last, but not least, the transmission of the changes acquired through 

 the mentioned factors to succeeding generations by heredity. It is 

 evident that Datwin based his views only upon facts available at that 

 time and known from observations of adult forms, but these facts 

 alone could not be considered as sufficient evidence for his views. 

 The theory itself, however, was so fascinating that a great num- 

 ber of enthusiastic investigators were induced to study the de- 

 velopment of individual animals, and the facts revealed by embryo- 

 logists at that period have been astonishing. It has been shown that 

 all metazoa develop from ova, that the ova of all animals undergo a 

 similar process of segmentation, and in every case a blastoderm is 

 formed, first consisting of a single layer, but consecutively changing 

 into one of two, and finally one of three layers of cells. The 

 similarity between these early stages in the development of widely 

 different animals has been found to be so striking that it is impos- 

 sible to distinguish one animal from another at this stage, and these 

 facts gave rise to the Gastrcea theory of Haeckel in support of the 

 views of Darwin. Haeckel considers that all forms of blastoderms, 

 consisting of two germinal layers, can be looked upon as modifica- 

 tions of the simple gastrula ; and as a gastrula is the foundation for 

 the development of a single individual ontogenesis so a simply con- 

 structed animal similar to it is to be considered as the ancestor of 

 all metazoa. He even constructed a treelike diagram to illustrate 

 how, from an undifferentiated being, gastrcea, by means of the above 

 mentioned factors pointed out by Darwin, an evolution to different 

 types, varieties, and species phylogenesis observed in the animal 



