Lamarck's name has been shadowed by that of Charles Darwin. And yet, Lamarck's Philosophie Zoologique was in modern times the first serious endorsement of the idea that animal species had evolved from one another. There had been ancient philosophers saying something similar, but these suggestions had never really caught on. Instead, the most respected theory in these matters was Aristotle's suggestion that plant and animal species had an inherent power to reproduce themselves – power that might not always work, due to deficiencies in the matter used, leading to the birth of monstrous degenerates – and that they had reproduced themselves forever, just like world had remained mostly similar for all eternity. The only novelty introduced to this theoretical framework was the Christian notion that the world was not eternal, but created by God, just like the first specimens of numerous animal and plant species.
Although the belief in the stability of plant and animal species ruled the field, this did not mean that all species would have been considered equal. On the contrary, animals as such were considered to be more perfect than plants, and human beings, if they were taken as part of animal kingdom, were considered as the apex and ruler of that kingdom. It was also common to evaluate different animal species according to their supposed proximity to humans as the most perfect specimen of animals.
Lamarck's view of organic beings was still closely related to the traditional view. Firstly, he made a sharp demarcation between plants and animals. Animals were characterised by irritability – a fashionable concept referring to the tendency of all animals to respond to stimuli. Lamarck was convinced that plants were characterised by non-irritability, and supposed counterexamples, like mimosa, just had developed mechanisms responding e.g. to weight of external objects. Because of this complete separation of plants and animals, the former did not, according to Lamarck, form a part of an evolutionary line of animals.
It is indeed a single evolutionary line Lamarck appears to have had in mind. His hierarchy of animal species is still quite traditionally one-dimensional, starting from microscopical animals and going through various levels of invertebrates to vertebrates, from fishes to reptiles, birds and finally mammals. Nowadays we know that this is not literally how animal evolution happened – birds did not generate mammals. Indeed, Lamarck's theories feel a bit artificial because of this one-dimensionality. For instance, he has to suppose that after the development of animals into insects, arachnids and crustaceans, this complex organisation devolved into worms, just because worms are in some important sense closer to vertebrates.
What has often been pointed out as a defect in Lamarck's system is his manner of describing the mechanism behind evolution. This is somewhat unfair, since even Darwin did not get it all right in one brushstroke, having no idea of the mechanism behind the inheritance of characteristics, which was discovered later by Mendel.
Lamarck starts from two quite true observations, firstly, that animals appear to transfer at least part of their characteristics to their progeny, and secondly, that individual animals tend to change their characteristics through habituation and influence of external circumstances. He then combines these observations into a hypothesis that through continual habituation and influence of circumstances a whole species will inevitably changes. For instance, because giraffes have for a long time reached for taller and taller trees, their necks have grown to their current size. Although a natural reading of Lamarck's ideas is that he supposed inquired characteristics to be inheritable, it could be that he just suggested that external circumstances and habits of species have a tendency to develop animal species into some direction. What the exact causal mechanism was behind this process was left for later zoologists to fill out.
What was really incredible was Lamarck's willingness to take the step toward making humanity part of the line of animal development. He especially considers chimpanzees as the animal closest in kind to humans and speculates about the possibility of a similar ape species habituating itself to walking upright, using its now free hands for grabbing tools and developing language, which gave the species tools for abstract thinking. Although Lamarck was thus relating humanity to animals, he still endorsed the notion that humans were the rulers of all other animals. Indeed, this special state was in Lamarck's theory not just a gift from God (although Lamarck does retain the ambiguous terminology of nature as a sort of active agent molding animal species), but a product of humanity's own endeavours.