254 THE PRESENT IMPORTANCE OF SCIENCE 



again begins with Leeuwenhoek. Before his day it was not 

 even known whether there existed in lowly organisms like 

 these mollusca anything comparable to the maleness and 

 femaleness recognized in higher forms. Their whole mode of 

 generation, whether spontaneous or by means of eggs, was a 

 matter of theoretical dispute. In his efforts to solve these 

 fundamental, but at the time wholly academic, questions, 

 Leeuwenhoek turned his microscope upon the fresh-water 

 mussels and discovered the innumerable eggs and larva 

 which crowd the brood-pouches of the females. These he 

 correctly interpreted as the young of the mussel in which 

 they were found ; and it is clear from his descriptions that he 

 saw enough of their development to justify his conclusion 

 that mussels, like the more familiar forms of animal life, 

 arose from eggs. 



In the subsequent advance of our knowledge two periods 

 are conspicuous, one marked by a mistaken hypothesis, the 

 other by the discovery of the parasitism. The first period 

 (1797 to about 1830) was ushered in when the failure to 

 secure stages beyond the glochidium led to the so-called 

 Glochidium-Theory, which maintained that the larvae were 

 not the young of the mussel from which they were obtained 

 but a wholly different species of bivalve living within the 

 mussel as a parasite. This theory had the negative advan- 

 tage of an incorrect hypothesis, it aroused opposition and 

 called forth investigations which showed once for all that the 

 glochidium was the young of the mussel in whose brood- 

 pouch it occurred. 6 



The collapse of the glochidium-theory left the subsequent 

 stages, by which the larvae reach the condition of miniature 

 adults, an unmapped territory where all trails went blind. 

 Not until 1866, when a young German investigator, Leydig, 



6 In passing, it is of interest that the word Glochidium, by which we still 

 designate these larvae, had its origin at the period when the supposed parasites 

 were described as a species parasitic upon the mussel and named Glochidium 

 parasiticum. 



