The samhita as anuvAkAs and anuvAkAs have sUktAs and sUktAs have the mantrAs
In the first manDala of the Rg vEda, there are 100 or more mantrAs by each RSi, so the RSis are referred to as s’atarcins.
Generally one sUkta addresses one dEvata. A sUkta may have any number of mantrAs.
The mantrAs of the first and tenth manDalAs are by many different RSis and are interesting – because you will find indra, agastya, yama, parAs’ara and others as RSIs of the mantrAs. I wonder why vEda vyAsa did this.
The 9th manDala is dedicated to to the pavamAna sOma dEvata and is again by many RSIs.
Vyasa has organised the other manDalAs by RSIs and their descendants.
5 by atri, 7 by vasiSTa, 3 by vis’vAmitra, 8 by kaNva, 2 is by gRtsamada, 4 by vAmadEva and gOtama, and 6 by bharadwAja.
- You can see by the above, that the mantrAs are not in chronological order. For chronology we look to the purANAs and itihAsAs.
Then again, in each manDala all the mantras by a given RSi have been organised by vyAsa on the basis of the dEvatAs. First agni, then indra, then vis’va dEvatAs, uSas and so on.
Within the agni sUkta of a RSI, you have his mantras in gAyatrI chandas first, then triSTup, then jagatI and others. The sUktAs with smaller number of mantrAs like 3, 4, 5 and 6 come first, followed by the longer sUktAs.
Sometimes you have sUktAs with different dEvatAs, different chandas and different RSIs too. And sometimes the same mantra can invoke more than one dEvata.
Reference : H.P. Venkata Rao.