392 HISTORY OF SCIENCE. 





the notochord, a slender rod which is precisely the embf^nic condi- 

 tion of the spinal column in the higher vertebrate animals. Thus if 

 Nature appears in any case to draw distinct lines of separation, it is 

 only that a gap occurs at that part of her otherwise insensibly gradated 

 series, and the progress of science tends continually to obliterate these 

 lines of demarcation instead of intensifying them, as would be the case 

 had Nature herself fixed barriers between living things by absolute dis- 

 tinctions. Further, the circumstance of so many different schemes of 

 classification having been successively brought forward, to the great 

 and increasing complication of science, proves that Nature has not 

 herself made those distinctions which for our convenience of study and 

 reference we find it necessary to make artificially and arbitrarily. This 

 is of course quite consistent with the fact that the productions of nature 

 have relations with each other with regard to which they fall into one 

 order of arrangement rather than into another. The continuity of the 

 animate world is strongly dwelt upon by Lamarck, but he does not 

 say that the existing races of animals form a simple linear and uniformly 

 gradated series ; he asserts that they form series ramifying and irre- 

 gularly gradated, and with a continuity not the less real because it is 

 not linear. The species with which each branch ends are connected 

 on one side at least with one species into which they shade off. In 

 all this there are no hypotheses, no suppositions. This gradation of 

 species is the great difficulty of the terminological zoologist and bota- 

 nist. When but few species of a given genus were known, it was easy 

 to distinguish them, but new discoveries are constantly filling the gaps 

 and obliterating the distinction, which at length can hardly be made 

 minute enough. Lamarck then points out that well-known facts show 

 that the individuals of a species are liable to be modified by the in- 

 fluence of surrounding circumstances, such as change of climate, soil, 

 food, etc. This modification may, he thinks, well proceed so far that 

 a naturalist would come to class the modified plants or animals as 

 different species. The seeds of the grass which flourishes in the moist 

 meadow maybe wafted on the slope of a neighbouring hill, from which 

 perhaps after in numberless successive generations its characters are 

 changed, its seeds may reach still higher ground, some dry and moun- 

 tainous region say, in which its characters undergo further change from 

 the original type flourishing in the lower and damper soil. From many 

 such considerations Lamarck concludes that the notion of fixed un- 

 variable species is an illusion. He enunciates as clearly and definitely 

 the hypothesis of evolution as applied to living things, as Laplace had 

 1 expressed the same doctrine in explanation of cosmical phenomena in 

 his celebrated nebular hypothesis. According to Lamarck, we may 

 gather from a general survey of the life of the globe, that organized 

 bodies are the products by natural operations acting through a long 

 space of time ; that nature commenced and is ever recommencing to 

 form the simplest organizations, these being the only organizations 



