At this point, however, Gould and Eldredge realized that their theory had a shattering consequence. If species actively work for their own conservation, and achieve it, we cannot say any more that they are continuously transforming themselves in order to express the features of higher taxa. This is precisely the mechanism invoked by phyletic gradualism in order to explain macroevolution, but we cannot have it both ways: stasis and phyletic gradualism are not compatible, and cannot both be true. And since it is stasis that is documented by the fossil record, Gould and Eldredge conclude that phyletic gradualism must be abandoned, even if this means that we no longer have a model for macroevolution.
The question, at this point, was que faire? When a widely accepted solution is found wanting, the need to replace it with an alternative model is strong, and Gould and Eldredge discovered that a way out is offered by the idea of species selection. If species are regarded as organic systems that are born, live and die as individuals do, one can conclude that a selection mechanism operates at the species level as it does at the individual level.
This solution takes the classical two birds with one stone. On the one hand, it is acknowledged that microevolution and macroevolution are two separate hierarchiacal levels, and therefore that each of them has its own independent laws. On the other hand, however, it is said that one level is ruled by the natural selection of individuals and the other by the natural selection of species, which means that all problems are solved by natural selection.
The explanation is ingenious, no doubt abut that. But is it true? Unfortunately we have no proof of this, and the mechanism of macroevolution remains therefore a mystery.

Where is biology going to?

The various fields that make up a science tend naturally to integrate, and with time such a process can lead to a true synthesis. Physics was the first science to achieve a synthesis of its disciplines, and it may be useful to compare that experience with its biological counterpart.