There is a more fundamental difference between Lamark’s theory of evolution and Darwin’s than the mechanisms they proposed (inheritance of acquired characteristics versus natural selection). Darwin, like biologists today, saw species as branching from a common root, where one species may differentiate into several others over time, and where many species (most even) eventually go extinct. This is fundamental to Darwinian evolution, and to explaining the original evidence that convinced him that evolution must be real. He realized it was the only reasonable way to explain the distribution of species around the world, especially, but not only, in isolated environments like the Galapagos. (Similar experiences studying the ecology of isolated islands, independently lead A.R. Wallace to the same conclusions a few years later.) Not only is Lamark unable to explain this data, it can’t be convincingly explained by the hypothesis (still the most popular at the time), that God created all the different species individually, and put them in the parts of the world to which they would be best adapted. (Only a schizophrenic God, if that, would distribute species geographically the way they actually are distributed.) Only after Darwin had come to the conclusion that evolution, of this branching, differentiating form, must be occurring, did he deliberately set out to find a mechanism that could be causing it. It took him a few more years hard work to come up with the idea of natural selection, and he would never have done that work if he had not already become convinced that branching, differentiating evolution had to be real.
Most evolutionists writing before Darwin (including, most notably, his grandfather Erasmus, and Robert Chambers) did not have his clear sense of evolution as a process of differentiation. Mostly they just liked the vague idea that living things were always changing and improving over time. Lamark’s conception was more specific, but it directly rejected the idea of evolution and speciation through differentiation that was central for Darwin (and for us). Lamark held that every species had its own independent origin as some sort of very simple organism at some point in the past, and gradually, over the course of time, evolved to become more complex and advanced. The more complex organisms around today, such as humans, were the older species who had had more time to evolve. Relatively simple species around today, earthworms, for instance, were younger species, created (or sontaneously generated from non-living matter) more recently, that had not had nearly so much time to evolve as yet. Lamark held that no species (or ‘lineages’) ever go extinct. He could not believe that a benevolent, omniscient God would create species only to let them become extinct, and, indeed, one of the main motivators for his theory was to explain away the evidence of fossils of types of organism that no longer seem to be around. According to Lamark, the explanation is not that they have died out, but that they have evolved into something else. The direct descendant of the T. Rex, the brontosaurus (yeah, I know!:rolleyes:) and the trilobite are, according to Lamark, still with us today, but looking very different.
In the context of this view of the structure of evolution, Lamark’s theory about the mechanism of evolution (inheritance of acquired characteristics) makes a lot of sense, and natural selection (which depends on the idea that most organisms are evolutionary dead ends, with their lineage doomed to go extinct) makes no sense at all. The trouble is that, though it explains fossils (in a way), it does not at all explain the evidence about the geographical distributions of species that had motivated Darwin.
The Darwinian evolutionary structure of differentiation and extinction, by contrast, explains not only the fossil record but also the biogeographical data that concerned Darwin (and, it eventually turned out, lots of other things too), and this is so even before we bring in the mechanism of natural selection. In fact, the Lamarkian inheritance of acquired characteristics mechanism is not particularly incompatible with the Darwinian evolutionary structure, and Darwin himself was prepared to believe that it might really happen and be an additional mechanism of evolution alongside natural selection. In fact, he gave this Lamarckian mechanism more and more credence in his later books, including later editions, published during his lifetime, of the Origin of Species. It was not shown to be unworkable until a much deeper understanding of genetics had been attained, in the early twentieth century.
Natural selection is important, and Darwin (and, independently, Wallace) discovered it. More fundamental, however, is the conception of evolution as a process of extinction driven differentiation, and that was also first clearly conceived of by Darwin, and, later and independently, by Wallace. It led both of them, fairly quickly, to the idea of natural selection as the mechanism driving it. Evolutionists before Darwin and Wallace did not understand the evolutionary process as being structured in this way. Most had no very clear sense of its structure at all, and Lamark’s structure was quite different and incompatible. As a consequence, their theories of evolution did not explain very much, and attracted few followers. Darwin’s theory explains a lot, but the lion’s share of that explanatory power (in its early decades, at least) came not from the idea of natural selection, but from the idea of evolution as a process of differentiation, where, like the first pair of finches blown by Pacific storms onto the Galapagos, one species can give rise to several more before it itself dies out.