The Living Machine 327 



Of all the features characteristic of living matter, none is 

 more so tjian reproduction. Attempts have, it is true, been 

 made to compare the growth of many crystals of salt in a 

 concentrating solution with this miracle of life, but such at- 

 tempts sound like a mere play upon words. There is noth- 

 ing in the inorganic world in any way comparable to this 

 wonderful phenomenon. Here then, if anywhere in the world 

 of life, we should find evidence of some force higher than 

 the physical forces, did any such exist. But what do we 

 find? We have seen in a previous chapter that the method 

 of reproduction (bi-sexual or parthenogenetic) can be altered 

 by external means; furthermore in Hydra it can similarly 

 be changed from asexual (budding) to sexual. In some 

 plants likewise the kind of reproduction may be determined 

 by external factors. But beyond the mere shifting of the 

 mode of reproduction by physical or chemical stimuli, it 

 has been found that the process of sexual reproduction itself 

 is a physico-chemical one and can be accomplished by arti- 

 ficial means. In the first place the attraction between the sex 

 cells is in some cases, though apparently not in all, a chem- 

 ical one. If a capillary glass tube containing a weak solu- 

 tion of malic acid (the acid found in apples and other fruits) 

 be placed in water containing the sperms of ferns and mosses, 

 the latter are attracted by the acid, and will enter the tube 

 in great numbers. The action here however may be similar 

 to that described above of " trapping" Paramoecium in a drop 

 of acid. With the spermatozoa of the sea urchin however 

 such chemical attraction appears not to exist. The union of 

 egg and sperm in cases where chemical attraction cannot be 

 proven appears to be due to chance. It is a well-known 

 fact that it is very difficult to cross different species of ani- 

 mals, this difference indeed being made the basis for a physi- 

 ological definition of species, those animals which breed to- 

 gether and produce fertile offspring being grouped as one 

 species ; and those which do not interbreed, or do not at least 

 produce fertile offspring being classed as distinct. In lower 

 animals union of egg and sperm of different species may be 

 prevented by physical differences such as size, or chemical 

 differences may prevent the development of an egg into which 

 by chance a foreign sperm has entered. In some cases it is 

 possible to fertilize the egg of species A with the sperm of 

 B, but the reciprocal cross is impossible. Among higher types 

 there appears to exist a mutual repugnance to union, which 

 effectually bars intermingling. Yet even here occasional in- 

 stances of crossing and the production of fertile offspring 

 are known, in crosses of hares and rabbits, various species of 

 fish, etc. Crosses between members of widely distinct groups 



