WHAT ARE SPECIES? 409 



as seen in c and d (Fig. 13). The coils grow closer together as the 

 filament increases in size, until a hollow cylinder is formed. From 

 the same threads below the base of the coil one or more processes 

 grow out which are the male organs, and, where one of these threads 

 reaches the tip of the female coiled filament, that organ is fertilized. 

 From this time on, this body undergoes various and complicated 

 changes, which finally result in a bright-yellow spherical body (Fig. 

 13,/), which consists of a thin wall inclosing a large number of sacs, 

 <7, each of which contains eight spores. 



To find all these various stages of development is a matter of some 

 time and patience; but nothing is more satisfactory in the study of 

 moulds than to trace all these steps, from the first bending of the fila- 

 ment to the perfect sphere with its multitude of spores. 



For a long time these two forms of fruit in the Aspergillus were 

 considered as belonging to distinct and widely-separated species, but, 

 when the microscope shows that they are produced from the same my- 

 celium, it is time to conclude that they are but two methods of continu- 

 ing the same species. 



Space forbids further details concerning our common moulds ; but 

 it is hoped enough has been said to show that among them the spe- 

 cies are distinct. The tiny forests which the microscope reveals are 

 made up of forms as decided as those which compose our woodlands 

 and groves. In closing, the reader expects an answer to the question 

 which very naturally arises, viz., " What good do they do ? " Though 

 often of great annoyance in domestic and other affairs, yet all in all it 

 is safe to say the good they accomplish far overbalances the harm. 

 They are scavengers which do in their own inobtrusive way a vast 

 amount of sanitary work. Though small in themselves, they are great 

 reducing agents, striving to bring about that equilibrium so necessary 

 to perfect harmony in the organic world. They hasten decay, tear 

 down the accumulating rubbish around us, and allow the elements thus 

 liberated to pass again into the cycle of ceaseless activity and growth. 



To the thoughtful mind moulds do not simply excite wonder or dis- 

 gust, but teach a deeper lesson of adaptation and service of little 

 things, in the perfect and economical scheme of creation. 



WHAT AKE SPECIES? 



By Prof. T. H. HUXLEY. 



IX its most general acceptation the word "species " signifies a kind 

 or sort of something, which something is the genus to which the 

 species belongs. Thus, a black stone is a species of the genus stone ; 

 a gray horse is a species of the genus horse ; a scalene triangle is a 

 species of the genus triangle ; and, generally, it may be said that 



